Load sensing hydraulic control system for variable displacement pump

A load sensing hydraulic control system for use in a work machine and adaptable for controlling the displacement of a variable displacement hydraulic pump. The control system includes a signal duplicating valve connected in fluid communication with both the pump controller and a fluid pressure source, and a sensor positioned and located for sensing the fluid pressure to the pump controller and outputting a signal to the controller indicative thereof. In response to signals received from the at least one sensor, the controller outputs a representative signal to the signal duplicating valve indicative of the highest pressure sensed by the at least one sensor, the signal duplicating valve being thereafter operable to allow fluid flow to pass therethrough to the pump controller.

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.

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.