Patent Publication Number: US-7905089-B2

Title: Actuator control system implementing adaptive flow control

Description:
TECHNICAL FIELD 
     The present disclosure relates generally to a control system and, more particularly, to an actuator control system that implements adaptive flow control. 
     BACKGROUND 
     Machines such as, for example, excavators, loaders, dozers, motor graders, and other types of heavy equipment use multiple actuators supplied with hydraulic fluid from an engine-driven pump to accomplish a variety of tasks. These actuators are typically pilot controlled such that, as an operator moves an input device, for example a joystick, an amount of pilot fluid is directed to a control valve to move the control valve. As the control valve is moved, a proportional amount of fluid is directed from the pump to the actuators. Various hydraulic control strategies have been implemented to control the amount of fluid flow between the pump and the actuators, including a load sensing control strategy. Load sensing control strategies measure a pressure differential between a maximum load pressure of a plurality of 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. Since variable displacement pumps are known to react slowly to load pressure changes, the pump is typically controlled to deliver fluid at an excessive pressure to ensure the maximum load pressure is available to the actuators. Hence, the pump is often required to deliver more pressure than necessary to overcome its own slow response to load demands. 
     One example of such a load sensing control system has been described in U.S. Pat. No. 5,129,230 (the &#39;230 patent) to Izumi et al. Specifically, the &#39;230 patent discloses a hydraulic control system implementing a variable displacement pump, two cylinders, two control valves, and an unloading valve. Additionally, the &#39;230 patent discloses a load pressure sensor for sensing the maximum load from the two cylinders, and a pump swash-plate position detector. Based on the sensed values from the load pressure sensor and the swash-plate position detector, a pressure difference between the pump and the maximum load is determined and transmitted to a controller. The controller instructs the variable displacement pump to deliver an excessive amount of pressure to ensure that the pump delivery pressure is greater than the maximum load pressure. An unloading valve is positioned between the pump and the control valves for holding the differential pressure less than a setting value. As a result, the &#39;230 patent is able to control a delivery rate of the pump when there are small or large pressure differences between the pump and the maximum loads. 
     Although load sensing pump control may, by itself, be adequate for some situations, at time it may be limited and inefficient. That is, pump control may be slow to respond to changes in required load pressure. And, pump control systems must maintain a relatively high amount of pressure differential to ensure that pump pressure is sufficient to meet the needs of the maximum load. These high pressures may place an unnecessary strain on the machine, whereby causing the pump to be overworked and the power source to inefficiently use fuel. 
     The disclosed actuator control system is directed to overcoming one or more of the problems set forth above. 
     SUMMARY OF THE INVENTION 
     In one aspect, the present disclosure is directed to an actuator control system. The actuator control system may include a pump and at least one actuator. The actuator control system may further include an actuator valve configured to control the at least one actuator. The actuator control system may also include a pump pressure sensor configured to determine a pump pressure value, and a load pressure sensor configure to determine a load pressure value. The actuator control system may additionally include a controller configured to receive the pump pressure value and the load pressure value. The controller may further be configured to compare the pump pressure value and the load pressure value, and selectively implement a primary control strategy and a secondary control strategy based on the comparison. 
     In another aspect, the present disclosure is directed to a method of controlling an actuator. The method may include sensing a pump pressure value and sensing a load pressure value. The method may further include comparing the pump pressure value and the load pressure value. The method may also include selectively implementing a primary control strategy and a secondary control strategy based on the comparison. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a side-view diagrammatic illustration of an exemplary disclosed machine; 
         FIG. 2  is a schematic illustration of an exemplary disclosed hydraulic control system for use with the machine of  FIG. 1 ; and 
         FIG. 3  is a flow diagram illustrating a method of operating the hydraulic control system of  FIG. 2 . 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  illustrates an exemplary machine  10 . Machine  10  may be a fixed or mobile machine that performs some type of operation associated with an industry such as mining, construction, farming, transportation, or any other industry known in the art. For example, machine  10  may be an earth moving machine such as an excavator, a dozer, a loader, a backhoe, a motor grader, a dump truck, or any other earth moving machine. Machine  10  may include a frame  12 , at least one work implement  14 , an operator station  16 , a power source  18 , and at least one traction device  20 . Power source  18  may drive the motion of traction device  20  and work implement  14  in response to commands received via operating station  16 . 
     Frame  12  may include any structural unit that supports movement of machine  10  and/or work implement  14 . Frame  12  may be, for example, a stationary base frame connecting power source  18  to traction device  20 , a movable frame member of a linkage system, or any other frame known in the art. 
     Work implement  14  may include any device used in the performance of a task. For example, work implement  14  may include a bucket, a blade, a shovel, a ripper, a dump bed, a hammer, an auger, or any other suitable task-performing device. Work implement  14  may be configured to pivot, rotate, slide, swing, or move relative to frame  12  in any other manner known in the art. 
     Operator station  16  may be positioned on machine  10  and include an operator interface device  22 . Operator interface device  22  may be configured to receive input from a machine operator indicative of a desired machine movement. It is contemplated that the input could alternately be a computer generated command from an automated system that assists the operator, or an autonomous system that operates in place of the operator. Operator interface device  22  may include a multi-axis joystick and be a proportional-type controller configured to position and/or orient work implement  14 , wherein a movement speed of work implement  14  is related to an actuation position of operator interface device  22  about an actuation axis. It is contemplated that additional and/or different operator interface devices may be included within operator interface station  16  such as, for example, wheels, knobs, push-pull devices, switches, and other operator interface devices known in the art. 
     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 known in the art. It is contemplated that power source  18  may alternatively be another source of power such as a fuel cell, a power storage device, and electric motor, or another source of power known in the art. 
     Traction device  20  may include tracks located on each side of machine  10  (only one side shown). Alternately, traction device  20  may include wheels, belts, or other traction devices. Traction device  20  may or may not be steerable. 
     As illustrated in  FIG. 2 , machine  10  may include a hydraulic system  24  having a plurality of fluid components that cooperate to move work implement  14  (referring to  FIG. 1 ) and/or to propel machine  10 . Specifically, hydraulic system  24  may include a tank  28  holding a supply of fluid, a pump  30  configured to pressurize the fluid and to direct the pressurized fluid to one or more hydraulic cylinders  36 A-C (only cylinders  36 A and  36 B are shown in  FIG. 2 ), one or more fluid motors (not shown), and/or to any other additional fluid actuator known in the art. Hydraulic system  24  may also include a control system  26  in communication with the fluid components of hydraulic system  24 . It is contemplated that hydraulic system  24  may include additional and/or different components such as, for example, accumulators, restrictive orifices, pressure relief valves, makeup valves, pressure-balancing passageways, and other components known in the art. 
     Tank  28  may constitute 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 fluid known in the art. One or more hydraulic systems within machine  10  may draw fluid from and return fluid to tank  28 . It is also contemplated that hydraulic system  24  may be connected to multiple separate fluid tanks. 
     Pump  30  may be configured to produce a flow of pressurized fluid and may include, for example, a variable displacement pump, a fixed displacement pump, or a variable delivery pump. Pump  30  may be drivably connected to power source  18  of machine  10  by, for example, a countershaft  34 , a belt (not shown), an electrical circuit (not shown), or in any other suitable manner. Alternatively, pump  30  may be indirectly connected to power source  18  via a torque converter, a gear box, or in any other appropriate manner. Pump  30  may vary displacement and/or delivery of hydraulic fluid. For example, a variable displacement pump may include an adjustable swash-plate (not shown) that may be electronically controlled based on operator input signals from operator input device  22  and/or machine input signals from various machine sensors (not shown) to allow variable control of pump output. It is contemplated that multiple pumps may be interconnected to supply pressurized fluid to hydraulic system  24 . 
     A flow rate available from pump  30  may be determined by sensing an angle of a swash-plate within pump  30  or by observing an actual command sent to pump  30 . It is contemplated that the flow rate available from pump  30  may alternatively be determined by a sensing device configured to measure an actual flow output from pump  30 . A flow rate available from pump  30  may be reduced or increased for various reasons such as, for example, to ensure that demanded pump power does not exceed available input power (from power source  18 ) at high pump pressures, or to vary pressures within hydraulic system  24 . 
     Hydraulic cylinders  36 A-C may connect work implement  14  to frame  12  (referring to  FIG. 1 ) via a direct pivot, via a linkage system with each of hydraulic cylinders  36 A-C forming one member in the linkage system, or in any other appropriate manner. Each of hydraulic cylinders  36 A-C may include a tube  38  and a piston assembly  40  disposed within tube  38 . One of tube  38  and piston assembly  40  may be pivotally connected to frame  12 , while the other of tube  38  and piston assembly  40  may be pivotally connected to work implement  14 . It is contemplated that tube  38  and/or piston assembly  40  may alternatively be fixedly connected to either frame  12  or work implement  14  or connected between two or more members of frame  12 . Each of hydraulic cylinders  36 A-C may include a first chamber  42  and a second chamber  44  separated by piston assembly  40 . First and second chambers  42 ,  44  may be selectively supplied with a pressurized fluid and drained of the pressurized fluid to cause piston assembly  40  to displace within tube  38 , thereby changing the effective length of hydraulic cylinders  36 A-C. The expansion and retraction of hydraulic cylinders  36 A-C may function to assist in moving work implement  14 . 
     Piston assembly  40  may include a piston  41  axially aligned with and disposed within tube  38 , and a piston rod  43  connectable to one of frame  12  and work implement  14  (referring to  FIG. 1 ). Piston  41  may include two opposing hydraulic surfaces, one associated with each of first chamber  42  and second chamber  44 . An imbalance of force on piston assembly  40  may cause piston assembly  40  to axially move within tube  38 . For example, a force resulting from a fluid pressure within first hydraulic chamber  42  acting on a first hydraulic surface being greater than a force resulting from the fluid pressure within second hydraulic chamber  44  acting on a second opposing hydraulic surface may cause piston assembly  40  to displace to increase the effective length of hydraulic cylinders  36 A-C. Similarly, when the resultant second force is greater than the resultant first force, piston assembly  40  may retract within tube  38  to decrease the effective length of hydraulic cylinders  36 A-C. 
     Each of hydraulic cylinders  36 A-C may include at least one proportional control valve  46  that functions to meter pressurized fluid from pump  30  to one of first and second hydraulic chambers  42 ,  44 , and at least one drain valve (not shown) that functions to allow fluid from the other of first and second chambers  42 ,  44  to drain to tank  28 . Proportional control valve  46  may include a spring biased proportional valve mechanism that is solenoid actuated and configured to move between a first position, at which fluid is allowed to flow into one of first and second chambers  42 ,  44 , and a second position, at which fluid flow is blocked from first and second chambers  42 ,  44 . The location of the valve mechanism between the first and second positions may determine a flow rate of the pressurized fluid directed into and out of the associated first and second chambers  42 ,  44 . The valve mechanism may be movable between the first and second positions in response to a demanded flow rate that produces a desired movement of work implement  14 . The drain valve may include a spring biased valve mechanism that is solenoid actuated and configured to move between a first position at which fluid is allowed to flow from first and second chambers  42 ,  44 , and a second position, at which fluid is blocked from flowing from first and second chambers  42 ,  44 . It is contemplated that proportional control valve  46  and the drain valve may alternately be hydraulically actuated, mechanically actuated, pneumatically actuated, or actuated in any other suitable manner. 
     Pump  30  may be in fluid communication with proportional control valves  46  via a hydraulic line  48 . Additionally, each proportional control valve  46  may be in communication with hydraulic cylinders  36 A-C via a hydraulic line  50 . 
     Hydraulic system  24  may also include a post compensating valve  52  and a check valve  54  associated with each hydraulic cylinder  36 A-C. It is contemplated that post compensating valve  52  and check valve  54  may serve to balance the load pressure between actuators and aid load sharing. More specifically, each post compensator valve  52  may be interconnected and operate with the same pressure differential. Therefore, the maximum load pressure of any one actuator may be applied to all actuators via post compensators  54 . In this manner, the velocity of all hydraulic cylinders  36 A-C may be substantially evenly reduced when pump output is insufficient to meet the demands of any one hydraulic cylinder  36 A-C. 
     Further, hydraulic system  24  may include a load sensing device  70 , for example, a shuttle valve for sensing the maximum fluid pressure of cylinders  36 A-C. Alternatively, load sensing device  70  may any known mechanism for identifying a maximum load pressure of a plurality of consumers. 
     Control system  26  may include a controller  56 . Controller  56  may be embodied in a single microprocessor or multiple microprocessors that include a means for controlling an operation of hydraulic system  24 . Numerous commercially available microprocessors can be configured to perform the functions of controller  56 . It should be appreciated that controller  56  could readily embody a general machine microprocessor capable of controlling numerous machine functions. Controller  56  may include a memory, a secondary storage device, a processor, and any other components for running an application. Various other circuits may be associated with controller  56  such as power supply circuitry, signal conditioning circuitry, solenoid driver circuitry, and other types of circuitry. 
     Controller  56  may be configured to receive input from operator interface device  22  and to control the flow rate of pressurized fluid to hydraulic cylinders  36 A-C in response to the input. Specifically, controller  56  may be in communication with each proportional control valve  46  of hydraulic cylinders  36 A-C via communication line  58 , and with operator interface device  22  via a communication line  60 . Controller  56  may receive the proportional signals generated by operator interface device  22  and selectively actuate one or more of proportional control valves  46  to selectively fill the first or second actuating chambers associated with hydraulic cylinders  36 A-C to produce the desired work tool movement. 
     Controller  56  may be in communication with a pump control device  32  via a communication line  62  and configured to change operation of pump  30  in response to a demand for pressurized fluid. Specifically, controller  56  may be configured to determine a flow rate of pressurized fluid that is required to produce machine movements desired by a machine operator (total desired flow rate) and indicated via operator interface device  22 . It is contemplated that a flow map (not shown) may be stored in memory of controller  56  and provides instructions to controller  56  for determining a required pump flow rate. The flow map may provide controller  56  with a required pump flow rate necessary to meet desired machine movement by the operator based on operator input signals and various machine input signals. Operator input may include signals from operator input device  22 . Machine input may include signals from position detectors (not shown) associated with control valves  46  indicating control valve position. Further, machine inputs may include signals indicative of limitations on pump  30  from other machine systems. For example, another machine signal may include a signal indicating the amount of torque available to pump  30 . In particular, a torque sensor (not shown) may transmit a signal to controller  56  indicating limited power source torque available to pump  30 . After receiving all operator and machine inputs, controller  56  may apply the flow map based on the input signals to send pump control device  32  a command of the required pump flow rate. Further, pump control device  32  may be electronically operated by controller  56 . 
     Control system  26  may include two pressure sensors, a pump pressure sensor  64  and a load pressure sensor  66 . Pump pressure sensor  64  may be located near pump  30  to monitor the pressure of fluid exiting pump  30 . Further, pump pressure sensor  64  may be in communication with controller  56  via communication line  68  to transmit pump pressure data to controller  56 . Load pressure sensor  66  may be in fluid communication with load sensing device  70  via hydraulic line  72 , whereby load sensing device  70  may permit passage of hydraulic fluid at a pressure equal to the maximum of the hydraulic cylinders  36 A-C. Further, load pressure sensor  66  may be in communication with controller  56  via communication line  74  to transmit the maximum load pressure data to controller  56 . Alternatively, control system  26  may include a differential pressure sensor (not shown) in place of, or in addition to, pump pressure sensor  64  and load pressure sensor  66 . 
     As determined by controller  56 , a function of the difference between a measured pump pressure value and a measured load pressure value may be defined as a margin pressure value. Therefore, margin pressure may serve as a measure of the excess fluid pressure generated by the pump to ensure that the actuators have sufficient fluid pressure. It may be desirable to set a margin range value including a lower range limit value (e.g., 500 KPa) and an upper range limit value (e.g., 2000 KPa). When the margin pressure value drops below the lower range limit value, operation of control system  26  may become less stable and less reliable. When the margin pressure exceeds the upper range limit value, operation of control system  26  may become inefficient. It is contemplated that the control system  26  may implement a primary control strategy that is pump regulated when the margin pressure value is within the lower and upper range limit values. Further, it is contemplated that the control system  26  may implement a secondary control strategy that is valve regulated when the margin pressure is outside the lower and upper range limit values. In other words, the primary control strategy may be implemented, under normal operating conditions, when a pressure differential between a pump pressure and a maximum load pressure is within a preset margin range. In contrast, a secondary control strategy may be selectively implemented when the pressure differential between the pump pressure and the maximum load pressure is outside the preset margin range. 
       FIG. 3  shows a flow-diagram illustrating a method of controlling hydraulic system  24  by implementing primary and secondary control strategies.  FIG. 3  will be discussed in detail in the following section. 
     INDUSTRIAL APPLICABILITY 
     The disclosed control system may be used in any machine where stable, reliable, and efficient hydraulic pressure control is a concern. The disclosed control system may regulate hydraulic fluid via a primary control strategy implementing pump control and a secondary control strategy implementing valve control. When a pressure differential between a pump pressure and a maximum load pressure are outside a preset margin range, the secondary control strategy may implement an actuator control system that may reduce the pressure differential to within the preset margin range. Operation of hydraulic control system  26  will now be described. 
     Regarding  FIG. 3 , control system  26  may begin regulation of the hydraulic system  24  at machine start-up. At start-up, the primary control strategy implementing pump control may be utilized (Step  76 ). Therefore, controller  56 , after receiving input signals, may access the stored flow map to determine the required pump flow rate based on operator input device  22 . 
     However, under certain conditions, the primary control strategy may be insufficient to meet system needs, and a secondary control strategy may be required. For example, when margin pressure closely approaches or exceeds the preset margin range (PMR), then a more responsive secondary control strategy may be required to meet the actuator pressure demands. Otherwise, hydraulic system  24  may not receive sufficient pump pressure to meet the maximum load of the hydraulic cylinders  36 A-C. 
     In order to determine when the secondary control strategy may be required, various system inputs may be received by controller  56 . For example, the pump pressure value (PPV) may be received from pump pressure sensor  64 , and the maximum load pressure value (LPV) may be received from load pressure sensor  66 . Pump pressure sensor  64  and load pressure sensor  66  may transmit the pump and the maximum load pressure values to controller  56  via communication lines  68  and  74 , respectively (Step  78 ). 
     Controller  56  may calculate the margin pressure value (MPV) as a function of the difference between the maximum load pressure value and the pump pressure value, and compare the margin pressure value to the preset margin range (Step  80 ). Based on the comparison, controller  56  may determine if the margin pressure value is within the lower and upper range limits of the preset margin range (Step  82 ). For example, if the preset margin range includes a lower range limit of 500 KPa and an upper range limit of 2000 KPa, then a margin pressure value of 1100 KPa is within the preset margin range. As in this situation, when the margin pressure value is within the preset range, controller  56  may determine if the secondary control strategy is currently being implemented (Step  86 ). If the secondary control strategy is currently being implemented, then controller  56  may suspend the secondary control strategy (i.e., revert back to the primary control strategy), because it may no longer be needed (Step  88 ). Alternatively, instead of suspending the second control strategy when the margin pressure value is within the preset margin range, it may be desirable to maintain the secondary control strategy as currently implemented to ensure that the margin pressure value remains within the preset margin range. Once controller  56  suspends the secondary control strategy or identifies that the secondary control strategy is not currently implemented, then controller  56  may continuously repeat steps  78 - 82  to determine if the secondary control strategy is required in response to changes in control system inputs. 
     However, if the preset margin range includes a lower range limit of 500 KPa and an upper range limit of 2000 KPa, then a margin pressure value determined to be 300 KPa may be outside the preset margin range and controller  56  may implement the secondary control strategy (Step  84 ). More specifically, controller  56  may determine if the margin pressure value is above or below the preset margin range (Step  90 ). In this situation, a margin pressure value of 300 KPa is below the lower range limit of 500 KPa and it may be desirable to increase margin pressure in order to ensure system reliability and stability. In other words, it may be desirable to increase margin pressure in order to ensure and maintain flow sharing between the loads. In order to increase the margin pressure, controller  56  may instruct control valves  46  to move toward a closed position (Step  92 ). Additionally, if the margin pressure is above the upper range limit, it may be desirable to decrease margin pressure in order to increase system efficiency. In order to decrease the margin pressure, controller  56  may instruct control valves  46  to move toward an open position (Step  94 ). Once the secondary control strategy has been implemented, controller  56  may continuously repeat steps  78 - 82  to determine if the secondary control strategy is still required in response to changes in control system inputs. 
     Controller  56  may instruct control valves  46  to open or close in proportion to the amount the margin pressure value is outside the preset margin range. For example, if the margin pressure value is only 50 KPa above the preset margin range upper limit value, then controller valves  46  may open a small amount to decrease margin pressure. In contrast, if the margin pressure value is 600 KPa above the preset margin range upper limit value, then controller valves  46  may open a large amount to decrease margin pressure more quickly. 
     During normal operation, when the margin pressure value remains within the preset margin range, pump control via the primary control strategy may be sufficient to maintain reliable, stable, and efficient hydraulic system control. Deviation from normal operation may occur when system disturbances, such as friction or other efficiency losses, cause the flow map to identify an improper match between pump output with a given control valve position. In this situation, control valves  46  may be controlled independent of pump  30  to adjust margin pressure. It is contemplated that the primary control strategy may be continuously implemented throughout operation of the system. Therefore, it may be preferable that the secondary control strategy operate in parallel with the primary control strategy. Hence, the primary control strategy and the secondary control strategy may be implemented independent of each another. For example, even when the margin pressure valve is outside the preset margin range, pump control may simultaneously be implemented in accordance with the flow map based on operator and system inputs. 
     Implementation of independently operated pump and actuator control strategies may provide a reliable, stable, and efficient hydraulic system control. Most notably, actuator control may improve hydraulic system control efficiency by reducing margin pressure necessary to ensure sufficient operation of a plurality of actuators. Hence, in addition to providing additional reliability and stability available from dual control strategies, improved efficiency may also be available from actuator control that is more responsive than ordinary pump control. 
     It will be apparent to those skilled in the art that various modifications and variations can be made to the disclosed control system without departing from the scope of the disclosure. Other embodiments of the control system will be apparent to those skilled in the art from consideration of the specification and practice of the control system disclosed herein. It is intended that the specification and examples be considered as exemplary only, with a true scope of the disclosure being indicated by the following claims and their equivalents.