Patent Publication Number: US-6216456-B1

Title: Load sensing hydraulic control system for variable displacement pump

Description:
TECHNICAL FIELD 
     This invention relates generally to load sensing hydraulic systems and, more particularly, to a load sensing hydraulic system which utilizes an external network for transferring a load pressure signal to a variable displacement pump. 
     BACKGROUND ART 
     The demand for better controllability and efficiency in work machine operations have lead to an increasing use of load sensing hydraulic systems. Compared to conventional hydraulic systems, load sensing hydraulic systems containing variable displacement pumps are more efficient since both the pump flow and the pump pressure are continuously matched to the actual load. Load sensing valve system configurations can be derived from both conventional closed-center and open-center type valves and a wide variety of different system configurations are being used. Different valve configuration yield different operational characteristics. Regardless of the particular valve configuration being utilized, it is always difficult to produce a load signal which is indicative of the actual load and which can be communicated to the pump controller without utilizing special load sensing valve mechanisms. It is also difficult to duplicate a true high pressure load sensing signal for communication with the pump controller without having a high pressure source associated therewith. 
     It is therefore desirable to provide a load sensing signal to the pump controller of a variable displacement hydraulic pump without utilizing special porting or other special valve means to mechanically control such signal, and without utilizing structure such as pressure compensating valves within the main control valve network to accomplish this task. It is also desirable to provide a mechanism for reducing or scaling down a high pressure load signal to a desired lower pressure load signal which will be representative of the actual load being experienced by the hydraulic system. 
     Accordingly, the present invention is directed to overcoming one or more of the problems as set forth above. 
     DISCLOSURE OF THE INVENTION 
     The present invention relates to a load sensing hydraulic control system for controlling the displacement of a variable displacement pump wherein the actual load or pressure exerted against an actuating cylinder used for controlling the movement of a work element or work attachment is sensed by a pressure transducer or other sensor means and a signal representative of the actual cylinder load is communicated to an electronic controller or other processing means. The electronic controller is operable to output a signal representative of the actual cylinder load to an electrohydraulic valve which acts as a signal duplicating valve for communicating a desired load signal to a variable displacement hydraulic pump so as to continuously adjust the displacement of the pump to control pump flow and pump pressure to match the actual cylinder load. In one aspect of the present invention, a charging valve is utilized to provide a minimum pump output flow rate and pressure to the pump and an accumulator is utilized to provide a source of pressurized fluid for generating an artificial load signal to the pump controller. In another aspect of the present invention, a pilot pump operating at a predetermined pressure is utilized to provide the desired artificial load signal to the pump controller. 
     The present load sensing system can be utilized with a wide variety of different types of main control valves such as a plurality of proportional valves, standard three position valves, split spool type valves, and other actuating valves coupled to appropriate actuators, motors or other devices for accomplishing a particular task where load sensing capability is desirable. The present system provides load sensing capability outside of the main control valve network, which design is less expensive, it includes fewer complex components, it saves wear and tear on the pump, and it provides a separate source for matching pump performance with the actual cylinder load. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     For a better understanding of the present invention, reference may be made to the accompanying drawings in which: 
     FIG. 1 is a schematic diagram of a load sensing hydraulic system constructed in accordance with the teaching of one embodiment of the present invention; and 
     FIG. 2 is a schematic diagram of a load sensing hydraulic system constructed in accordance with the teachings of another embodiment of the present invention. 
    
    
     BEST MODE FOR CARRYING OUT THE INVENTION 
     Referring to FIG. 1, a load sensing hydraulic pump pressure control system  10  is shown in combination with a variable displacement pump  12  which is connected in fluid communication with a tank  14  and a hydraulic cylinder or other work element  16  through a discharge passage  18 . The hydraulic pump  12  includes a pump displacement controller  20  which is connected to a displacement control element  22 , the pump controller  20  receives a load sensing signal via fluid path  24  so as to adjust the displacement control element  22  to achieve and maintain a desired fluid pressure to the actuating cylinder  16  in response to the load sensing signal. It is recognized and anticipated that the pump  12  and its associated controller  20  can take on a wide variety of different configurations depending upon the particular system application involved and the controller  20  may include a spring or some other biasing mechanism which will resiliently bias the displacement control element  22  to either its maximum or minimum displacement setting. The pump  12  will adjust the displacement control element  22  in response to the load sensing signal received via fluid path  24  in order to achieve a desired fluid flow through the discharge passageway  18 . 
     In the embodiment illustrated in FIG. 1, a main control valve mechanism  26  for controlling the operation of the actuating cylinder  16  includes four separate proportional electrohydraulic valves  28 ,  30 ,  32  and  34 , which valves move the actuating cylinder  16  incrementally based upon signal inputs from an electronic controller or processor  38 . Each valve  28 ,  30 ,  32  and  34  is electrically controlled via processor or controller  38  based upon operator commands inputted to processor  38  via an operator control mechanism  40  such as one or more control levers or joysticks associated with a particular work machine. Movement of the operator input device  40  outputs appropriate signals to controller  38  via conductive path  42  and, based upon such input signals  42 , controller  38  controls the operation of proportional valves  28 ,  30 ,  32  and  34  by outputting appropriate signals via conductive paths  44 ,  46 ,  48  and  50  to the solenoids or other electrical actuator means  52 ,  54 ,  56  and  58  associated respectively therewith. In this regard, valve  28  controls fluid flow from pump  12  via discharge passage  18  to the head portion  60  of actuating cylinder  16  via fluid path  62 ; valve  30  controls the discharge of fluid from the head end portion  60  of actuating cylinder  16  to tank  14  via fluid paths  62  and  64 ; valve  32  controls the discharge of fluid from the rod end portion  66  of actuating cylinder  16  to tank  14  via fluid paths  68  and  70 ; and valve  34  controls fluid flow from pump  12  to the rod end portion  66  of actuating cylinder  16  via fluid paths  18  and  68 . 
     Control valves  28 - 34  operate in a conventional manner such that when the operator commands the actuating cylinder  16  to extend via operator input device  40 , the controller or processor  38  outputs appropriate signals to close valves  30  and  34  and open valves  28  and  32  thereby allowing fluid flow from pump  12  to travel through valve  28  to the head end portion  60  of actuating cylinder  16  causing the cylinder to extend. As cylinder  16  extends, the fluid present in the rod end portion  66  is allowed to return to tank  14  through valve  32 . In a similar manner, if the operator commands the actuating cylinder  16  to retract via operator input device  40 , the controller or processor  38  will output appropriate signals to close valves  28  and  32  and open valves  30  and  34  such that fluid flow will be directed through valve  34  to the rod end portion  66  of actuating cylinder  16  thereby causing the cylinder to retract. As cylinder  16  retracts, the fluid present in the head end portion  60  is allowed to return to tank  14  through valve  30 . Pressure sensors  72  and  74  are coupled respectively to fluid paths  62  and  68  and sense the fluid pressure being exerted against the head and rod end portions of the actuating cylinder  16  respectively. When the actuating cylinder  16  is under load, the pressures sensed by sensors  72  and  74  represent the actual cylinder load. This actual cylinder load or pressure is communicated to controller or processor  38  from the respective sensors  72  and  74  via conductive paths  76  and  78  respectively. As a result, controller or processor  38  continuously receives a load sensing signal indicative of the actual load or pressure associated with actuating cylinder  16 . 
     The present pump load sensing control system  10  further includes an accumulator  80 , a charging valve  82 , another electrohydraulic valve  84 , another pressure sensor  86 , a resolver  88 , and a pair of check valves  90  and  92  as illustrated in FIG.  1 . These components form an external network separate and apart from the main control valve mechanism  26  for providing a desired load sensing signal to pump  12  as well be hereinafter explained. The accumulator  80  is provided as a pressure source for providing fluid flow through valve  84 ; charging valve  82  is provided to insure that a minimum pressure load is set for pump  12 ; and the electrohydraulic valve  84  is provided as a signal duplicating valve so that an artificial load signal of lower pressure can be provided to the pump controller  20  to control and regulate the fluid pressure to the actuating cylinder  16  based upon the actual cylinder load being sensed by sensors  72  and  74 . In this regard, accumulator  80  is connected in fluid communication with the inlet port  85  of valve  84  via fluid path  98  and the outlet port  87  of valve  84  is connected in fluid communication with pump controller  20  via fluid paths  108 ,  103  and  24 . The charging valve  82  has an inlet port  83  connected in fluid communication with pump  12  and the accumulator  80  and an outlet portion  89  connected in fluid communication through resolver  88  with the pump controller  20 . Charging valve  82  is provided for use only during the initial charging of accumulator  80  as will be hereinafter explained. 
     Accumulator  80  is initially charged by pump  12  via fluid paths  94 ,  96  and  98 . While accumulator  80  is charging to a predetermined charge pressure, fluid will flow through check valve  90  to accumulator  80  as well as through fluid path  94  to the charging valve  82 . Fluid will continue to flow through charging valve  82  and through resolver  88  back to the pump controller  20  via fluid paths  103  and  24 . As accumulator  80  is being charged, a pressure signal is being provided to charging valve  82  via fluid path  100 . When accumulator  80  is charged to a predetermined charge pressure, the pressure signal provided to charging valve  82  via fluid path  100  acts against the spring or biasing means  102  of valve  82  to close valve  82  at fluid path  94 . In this regard, the spring or biasing mechanism  102  will be set so as to close valve  82  when accumulator  80  is charged to a predetermined charge pressure. When valve  82  closes, no fluid flow via flow path  94  will reach resolver  88  and accumulator  80  will be providing fluid flow to valve  84  for use as will be hereinafter explained. The load signal inputted to pump controller  20  via fluid paths  103  and  24 , once charging valve  82  closes and while system  10  is operating under a no load condition will be a signal representative of some minimum pump output flow level. Charging valve  82  therefore sets pump  12  at some minimum predetermined flow and pressure level based upon the predetermined charge pressure of accumulator  80  which will close valve  82 . This minimum flow and pressure level of pump  12  can be changed by changing the predetermined charge pressure of accumulator  80  which will close valve  82 . Once charging valve  82  closes, accumulator  80  will be constantly charged by pump  12  via fluid paths  94 ,  96  and  98 . 
     When the operator inputs a signal to controller  38  via input device  40  to control the operation of actuating cylinder  16 , sensor  72  or  74  will sense the actual load pressure being exerted on actuating cylinder  16  depending upon whether the cylinder is being extending or retracted, and such load sensing signal will be communicated to controller  38  as previously explained. Based upon the actual load condition of cylinder  16 , controller  38  will output a signal to valve  84  via conductive path  106  so as to incrementally open valve  84  thereby allowing fluid under pressure from accumulator  80  to flow therethrough via flow paths  108 ,  103  and  24  to pump controller  20 . This fluid flow from valve  84  to pump controller  20  is an artificial load sensing signal designed to match the actual load or pressure being experienced by actuating cylinder  16  as communicated via sensors  72  and  74 . In this regard, controller  38  will output a signal to valve  84  representative of the highest load pressure being sensed by sensors  72  and  74 . 
     Controller  38  is programmed to output an appropriate signal to valve  84  to proportionately open valve  84  so as to provide an appropriate load sensing signal to pump controller  20  to either increase or decrease the flow pressure to actuating cylinder  16  so as to match the load. In this regard, the pressure sensor  86  positioned in communication with flow path  108  will continuously output a signal to controller  38  indicative of the load sensing pressure being inputted to pump controller  20 . When such load sensing signal reaches the appropriate desired pressure level as programmed into controller  38 , controller  38  will output an appropriate signal to valve  84  to incrementally control such valve so as to maintain the appropriate load sensing signal to pump controller  20 . In other words, valve  84  will hover and maintain the appropriate load sensing signal to match the actual cylinder load in response to signals inputted to controller  38  from sensors  72  and  74 . The load sensing signal being provided through valve  84  is a signal which produces a substantially reduced pressure flow to pump controller  20  as compared to the actual operating pressures being exerted on actuator cylinder  16 . Electrohydraulic valve  84  therefore acts as a signal duplicating valve which, in conjunction with accumulator  80 , provides a more desirable pressure reduced load sensing signal to pump controller  20 . 
     When hydraulic system  10  is under load, accumulator  80  will be constantly charged by pump  12  via flow paths  94 ,  96  and  98  and charging valve  82  will remain closed. Charging valve  82  is only operational during initial charging of accumulator  80 . As a result, the load sensing signal provided to pump controller  20  via valve  84  will always be a representative signal to match the load or pressure being experienced by cylinder  16  and such signal will be a reduced pressure signal controlled by controller  38  via inputs from pressure sensor  86 . Check valve  92  is provided in flow path  98  so as to prevent any feed back flow to accumulator  80 . 
     FIG. 2 illustrates another load sensing pump control system  110  wherein the proportional control valves  28 ,  30 ,  32  and  34  have been replaced with a conventional three position valve  112  and wherein the accumulator  80 , charging valve  82 , resolver  88 , check valve  90  and the plumping associated with such components have been replaced by a pilot pump  114  operating at a predetermined pressure. In all other respects, the load sensing pressure control system  110  illustrated in FIG. 2 operates in substantially the same manner as previously described with respect to the control system  10  illustrated in FIG.  1 . 
     For example, based upon an operator command inputted through operator input device  40 , the controller or processor  38  will output an appropriate signal to the actuating solenoids or other actuating means  116  and  118  associated with valve  112  via conductive paths  120  and  122  to control movement of the actuating cylinder  16  in the appropriate direction. If valve actuating means  118  is actuated, fluid flow from pump  12  will be directed to the head portion  60  of actuating cylinder  16  via fluid paths  18  and  124  so as to extend the cylinder  16  and fluid present in the rod end portion  66  will be allowed to exit and travel to tank  14 . In similar fashion, if valve actuating means  116  is actuated, fluid flow from pump  12  via fluid path  18  will be allowed to travel to the rod end portion  66  of actuating cylinder  16  via fluid paths  18  and  126  so as to retract the cylinder and any fluid present in the head portion  60  will be allowed to exit and travel to tank  14 . Here again, pressure sensors  72  and  74  are coupled respectively to fluid paths  124  and  126  and sense the actual load or pressure being exerted on actuating cylinder  16 . Sensors  72  and  74  likewise continuously communicate with controller  38  and input signals thereto via control paths  76  and  78  indicative of the actual load or pressure being experienced by cylinder  16 . Based upon these actual load sensing signals, controller  38  outputs an appropriate signal via conductive path  106  to the signal duplicating valve  84  to again send a desired load sensing signal of reduced pressure to pump controller  20  via fluid path  128  to again adjust and change the pump displacement control element  22  so as to output the necessary flow to match the actual load or pressure being exerted against actuating cylinder  16 . 
     Instead of accumulator  80  (FIG. 1) providing the fluid flow source to valve  84 , a pilot pump  114  connected in fluid communication with valve  84  via fluid path  127  is provided to accomplish this task. Pilot pump  114  operates at a predetermined pressure which is preferably lower than the operational pressure provided to actuating cylinder  16  via pump  12 , and further provides a reduced pressure or artificial load sensing signal via fluid path  128  to pump controller  20  when proportional valve  84  is incrementally actuated. Here again, the signal outputted by controller  38  to valve  84  will be a representative signal to adjust the displacement of pump control element  22  to match the highest actual load or pressure being sensed by sensors  72  and  74  and pressure sensor  86  will communicate this representative pressure signal to controller  38  via conductive path  104 . A relief valve  130  is provided to control the maximum fluid pressure to valve  84  via fluid path  127 . Here again, as the actual load or pressure to actuating cylinder  16  changes, such actual load changes are communicated to controller  38  via sensors  72  and  74 , and controller  38  will output an appropriate signal to valve  84  to provide a desired load sensing signal to pump controller  20 . 
     This embodiment further reduces the number of components used in the external network to provide the desired load sensing signal and it provides a more controllable mechanism for providing fluid flow to valve  84  since the output flow and pressure from pilot pump  114  to valve  84  can be easily established and maintained. 
     INDUSTRIAL APPLICABILITY 
     As described herein, the present load sensing hydraulic control system has particular utility in a wide variety of different applications including utility in a wide variety of different work machines and other vehicles wherein actuating cylinders, motors, or other actuators or work elements are being controlled by one or more variable displacement hydraulic pumps, and wherein load sensing capability is desirable. In the present load sensing system, an artificial load sensing signal of reduced pressure is provided to the pump controller so as to change the output flow from the pump to match the actual load or pressure being exerted against the actuating cylinder  16  or some other work element. This arrangement reduces the wear and tear on the variable displacement pump and provides an improved pressure control system which is separate and apart from the main control valve structure such as the valves  28 - 34  illustrated in FIG.  1  and valve  112  illustrated in FIG.  2 . As a result, the pump controller  20  is responsive to the actual load or control pressure being exerted against actuating cylinder  16 . 
     Although there has been illustrated and described herein two specific embodiments of a load sensing control system for use with a variable displacement hydraulic pump incorporating the principles of the present invention as illustrated in FIGS. 1 and 2, it is clearly understood that the hydraulic system embodiments of FIGS. 1 and 2 are merely for purposes of illustration only and that changes and modifications may be readily made to the overall circuit configuration by those skilled in the art without departing form the sprit and scope of the present invention. For example, besides being operable with a plurality of proportional electrohydraulic valves such as valves  28 - 34  (FIG.  1 ), or a conventional three position control valve  112  (FIG.  2 ), it is recognized and anticipated that the present load sensing control system can be utilized with a wide variety of other types of main control valves such as split spool type valves and the like. Also, importantly, it is also recognized and anticipated that the present load sensing system could be coupled to a plurality of different main control valves, the signal duplicating valve  84  being controlled in response to the highest actual load or pressure being sensed by any one of a plurality of pressure sensors such as sensors  72  and  74 . 
     Still further, the various pressure sensors  72 ,  74  and  86  used in the present control systems are well known in the art and a wide variety of different types of pressure sensors may be utilized. It is also recognized and anticipated that other means and methods may be used to determine the flow pressures associated with the actuating cylinder  16  via fluid paths  62 / 124  and  68 / 126  and with the pump  12  via fluid path  18 . 
     It is also recognized that electronic controllers or processors such as controller  38  are commonly used in association with a wide variety of hydraulic systems, particularly in work machines, for accomplishing various tasks. Controller  38  may typically include processing means such as a microcontroller or microprocessor, associated electronic circuitry such as input/output circuitry, analog circuits or programmed logic arrays, as well as associated memory. Controller or processor  38  can therefore be programmed to sense and recognize the appropriate signals indicative of the various pressure conditions being sensed by sensors  72  and  74  and, based upon such sensed conditions, controller or processor  38  will provide appropriate output signals to valve  84  to control the output flow of the variable displacement pump  12 . 
     Other aspects, objects and advantages of the present invention can be obtained from a study of the drawings, the disclosure and the appended claims.