Method and system for controlling fluid flow in an electrohydraulic system having multiple hydraulic circuits

A method and system for controlling fluid flow from a hydraulic fluid supply via a single hydraulic pump in an electrohydraulic system having multiple hydraulic cylinders connected to the hydraulic pump and corresponding work implements includes input devices for generating input signals representative of a desired amount of movement of at least two of the work implements. A controller, coupled to the input devices and the cylinders, determines a scaling factor for each of the hydraulic circuits associated with each of the cylinders for compensating the hydraulic circuit for receiving less than maximum fluid flow from the pump. The controller also determines a desired percentage of fluid flow to each of the hydraulic circuits based on the desired amount of movement of the work implements and the corresponding scaling factors and controls the fluid flow from the fluid supply to each of the hydraulic circuits based on the desired percentages of fluid flow to allow for maximum fluid flow to each of the hydraulic circuits.

TECHNICAL FIELD
 This invention relates generally to a method and system for controlling
 fluid flow in an electrohydraulic system and, more particularly, to a
 method and system for controlling the flow of fluid in an electrohydraulic
 system having multiple hydraulic circuits coupled to a single hydraulic
 pump.
 BACKGROUND ART
 Work machines such as wheel type loaders include work implements capable of
 being moved through a number of positions during a work cycle. Such
 implements typically include buckets, forks, and other material handling
 apparatus. Control levers are mounted at the operator's station and are
 connected to hydraulic circuits associated with each of the implements for
 moving the implements. The operator must manually move the control levers
 to open and close hydraulic valves that direct pressurized fluid from a
 hydraulic pump to hydraulic cylinders that in turn cause the implements to
 move.
 Today, more and more hydraulic circuits are being driven electronically via
 a solenoid. That is, rather than be driven by fixed mechanical linkages,
 the hydraulic cylinders are driven by solenoids that are actuated via
 electronic signals from a microprocessor-controlled controller. Thus, the
 hydraulic cylinders are controlled via pre-programmed control logic.
 However, when changes are made to such an electrohydraulic system that
 affect flow characteristics, such as hose dimensions, orifice sizing,
 valve conversions, etc., the control logic must be revised accordingly in
 order to maximize fluid flow.
 Accordingly, it is an object of this invention to provide a method and
 system that automatically calibrates software control in order to maximize
 hydraulic fluid flow to hydraulic cylinders in an electrohydraulic system.
 The present invention is directed to overcome one or more of the problems
 as set forth above.
 DISCLOSURE OF THE INVENTION
 In one aspect of this invention, a method is provided for automatically
 controlling fluid flow in an electrohydraulic system having multiple
 hydraulic cylinders, each connected to a single hydraulic pump and a
 corresponding work implement and having at least two hydraulic circuits
 associated therewith for receiving the fluid from a supply and moving the
 cylinders accordingly. The method begins with determining a scaling factor
 for each of the hydraulic circuits for compensating the hydraulic circuit
 for receiving less than maximum fluid flow from the pump. An input signal
 is received representing a desired amount of movement of at least two of
 the work implements, and a desired percentage of fluid flow to each of the
 hydraulic circuits associated with each of the cylinders is determined
 based on the desired amount of movement of the work implements and the
 scaling factor of the corresponding hydraulic circuits. Finally, the
 amount of fluid flow from the fluid supply to each of the hydraulic
 circuits is controlled based on the corresponding desired percentage of
 fluid flow so as to allow for maximum fluid flow to each of the hydraulic
 circuits.
 In another aspect of the invention, a system is provided for carrying out
 the steps of the above-described method. The system includes a hydraulic
 fluid supply coupled to a plurality of hydraulic cylinders via a single
 hydraulic pump. Each of the cylinders are coupled to corresponding work
 implements, and have at least two hydraulic circuits associated therewith
 for receiving the fluid supply and moving the cylinders accordingly. The
 system further includes at least two input devices for generating at least
 two corresponding input signals representative of a desired amount of
 movement for at least two of the work implements. The system also includes
 a controller, coupled to each of the input devices and each of the
 cylinders, for determining a scaling factor for each of the hydraulic
 circuits for compensating the hydraulic circuit for receiving less than
 maximum fluid flow from the pump, determining a desired percentage of
 fluid flow to each of the hydraulic circuits associated with each of the
 cylinders based on the desired amount of movement of the work implements
 and the scaling factor of the corresponding hydraulic circuits, and for
 controlling the amount of fluid flow from the hydraulic fluid supply to
 each of the hydraulic circuits based on the corresponding desired
 percentage of fluid flow.

BEST MODE FOR CARRYING OUT THE INVENTION
 Turning now to FIG. 1, there is shown a schematic block diagram of the
 electrohydraulic system of the present invention, denoted generally by
 reference numeral 10. The system 10 includes hydraulic cylinders 12
 coupled to their corresponding work implements 14. Although only two
 cylinders 12 and corresponding work implements 14 are shown, the present
 invention applies to electrohydraulic systems employing a plurality of
 cylinders 12 and work implements 14.
 Each of the cylinders 12 include a first hydraulic circuit 16 and a second
 hydraulic circuit 18, such as a head side and a rod side, for extending
 and retracting the cylinder 12. Activation of either circuit 16, 18 is
 controlled via movement of a valve 20, such as a spool valve. As the valve
 20 moves in one direction or another, more fluid flows to one hydraulic
 circuit or the other, resulting in more movement in either the extending
 or retracting position.
 The valves 20 are each coupled to a single hydraulic pump 22 via fluid
 inlets 24. The hydraulic pump 22, powered by a power source 26, such as a
 battery, pumps hydraulic fluid from a hydraulic fluid supply 28 through
 supply inlet 30 into each of the cylinders 12 via the fluid inlets 24.
 Each of the valves 20 is also coupled to hydraulic fluid supply 28 via
 fluid outlets 32 for returning the hydraulic fluid back to the hydraulic
 fluid supply 28.
 Thus, extension and retraction of the cylinders 12 is controlled via the
 amount of hydraulic fluid passed by the valve 20 to the first and second
 hydraulic circuits 16, 18. The amount of fluid flow into each of the
 cylinders is determined based on the desired movement of each of the work
 implements 14, which is governed by the amount of motion applied to input
 devices 34 by a user. Input device 34 may be a joystick, lever, or any
 other similar device or combination of these type of devices.
 Rather than being mechanically linked directly to the cylinders 12, the
 input devices 34 are coupled to a controller 36 having control logic
 programmed therein, which is in turn coupled to a solenoid 38 associated
 with each of the valves 20. Controller 36 controls the amount and polarity
 of current applied to each solenoid 38 thereby controlling movement of
 each of the valves 20. Positions sensors 40 are coupled to each of the
 cylinders 12 for sensing the position of the cylinder 12 and generating a
 position signal for receipt by controller 36. Controller 36 utilizes the
 position information in controlling the movement of the valves 20 via the
 solenoids 38, as will be described in greater detail below.
 FIG. 2 is a flow diagram illustrating the general steps associated with the
 control portion of the method of the present invention. First, the
 controller 36 automatically calibrates the electrohydraulic system 10 by
 determining a scaling factor for each of the hydraulic circuits 16, 18, as
 shown at block 50. This scaling factor is later applied when determining a
 desired fluid flow to each of the hydraulic circuits 16, 18 to compensate
 the hydraulic circuits for receiving less than maximum fluid flow from the
 one pump 22.
 The steps performed in determining the scaling factors are illustrated in
 the flow diagram shown in FIG. 3. First, a plurality of fluid flows for
 each of the hydraulic circuits is determined representative of the amount
 of fluid flowing into the hydraulic circuit in response to various
 different currents applied to the solenoid 38, as illustrated at blocks
 52-56. To obtain the fluid flows, a plurality of currents are applied to
 each of the hydraulic circuits 16, 18 via the associated solenoids 38, as
 shown at block 52. The currents are preferably ramped up within a
 predetermined current range and ramped down, or reversed in polarity, to
 account for movement of the valve 20 in both directions.
 In response to the application of each of the currents, the position of the
 corresponding cylinder 12 is sensed via the cylinder's position sensor 40,
 as shown at block 54. Position sensor 40 transmits the position
 information to the controller 36 for processing. From this information,
 controller 36 can then determine the amount of fluid flowing into each of
 the hydraulic circuits 16, 18.
 This is accomplished by first differentiating the position information to
 obtain a velocity signal, V, for the cylinder 12 at each current value, C.
 That velocity signal, V, is then multiplied by the area, A, of the
 corresponding hydraulic circuit 16, 18 to obtain a fluid flow value, Q.
 That is, Q=V.times.A. Thus, for each current value, C, a corresponding
 fluid flow, Q, is determined based on the differentiated position signal
 and the area. FIGS. 4-7 are tables illustrative of the type of flow
 mapping that is performed to obtain the fluid flows for each of the
 hydraulic circuits. Although application of five current values is
 illustrated in FIGS. 4-7, it should be appreciated that the present
 invention is not limited to only five current values but may be more or
 less, depending on the application.
 At this time, an averaging of some of the fluid flows may be done to
 account for hysteresis associated with the valves 20. That is, upon an
 initial increase in current, the valve 20 is sluggish in opening, and the
 same is true during an initial decrease in current. Thus, the fluid flows
 may be averaged over two or more of the currents that is indicative of the
 valve 20 opening and/or closing. For example, in FIG. 4, fluid flows a and
 b may be averaged to obtain a new fluid flow associated with current C=1.
 Upon determining all of the fluid flows for each of the hydraulic circuits
 16, 18, the method proceeds to determine corresponding percentages of
 fluid flows, Q%, as shown at block 58. Each of the fluid flows are divided
 by the maximum fluid flow for that hydraulic circuit and multiplied by
 100. For example, the maximum fluid flow for hydraulic circuit ("H. C.")
 #1 in FIG. 4 is Q=e at C=5. Thus, each of the fluid flows is divided by e
 and then multiplied by 100% to determine a corresponding percentage of
 fluid flow.
 Finally, the scaling factors are determined, block 60, based on the maximum
 fluid flows. Each of the maximum fluid flows are compared to the maximum
 fluid flow of all the hydraulic circuits to obtain a ratio indicative of
 the percentage of maximum flow each hydraulic circuit receives. For
 example, if the maximum flow for each of the circuits shown in FIGS. 4-7
 are e, j, o, and t, respectively, and o is the highest amount of fluid
 flow out of all of the fluid flows, the scaling factor is determined by
 dividing o by e, j, and t to get the scaling factor for H. C. #1, H. C.
 #2, and H. C. #4, respectively. Of course, the scaling factor for H. C.
 #3, is 1.0 since it receives maximum flow.
 Now that the scaling factors are determined, the controller 36 utilizes
 these factors in modifying input commands to each of the hydraulic
 circuits 16, 18. Returning now to FIG. 2, the method proceeds to determine
 a desired amount of movement for the work implements 14, as shown at block
 62, in determining the appropriate input commands to the cylinders 12.
 This is accomplished by receiving and processing input commands
 transmitted by input devices 34 associated with each of the work
 implements 14.
 According to a predetermined algorithm stored in controller 36, controller
 36 determines desired velocities for moving each of the cylinders 12
 associated with the work implements 14. However, since more than one work
 implement 14 is being moved at one time and only one pump 22 is available
 to supply the fluid flow, controller 36 determines a relative motion, or
 velocity, for moving each of the cylinders 12, as shown at block 64. This
 relative velocity is determined by dividing each of the desired velocities
 by the sum of all the desired velocities.
 From the desired relative velocity to be applied to each cylinder 12,
 controller 36 determines a desired percentage of fluid flow to each of the
 hydraulic circuits, as shown at block 66. Again, the desired percentage of
 fluid flow can be determined by multiplying the relative velocity by the
 area of the hydraulic circuit.
 The controller 36 then controls the fluid flow to each of the circuits
 based on the desired percentage of fluid flow, as shown at block 68. This
 is accomplished utilizing the tables shown in FIGS. 4-7. For example, if
 the desired percentage of fluid flow to H. C. #1 equals (b/e) .times.100%,
 then controller 36 applies current C=2 to the appropriate solenoid 38. If
 the desired percentage of fluid flow does not exactly match any of the
 percentages of fluid flow in the table, controller 36 performs an
 interpolation to determine the desired amount of current.
 The method can proceed to perform closed loop control if desired, as shown
 at conditional block 70. In closed loop control, controller 36 uses the
 position information from position sensor 40 to update the current command
 to the solenoids 38, as shown at block 72. From the position signal, an
 actual flow to 30 the hydraulic cylinder, block 74, is determined by
 differentiating the position signal to determine the velocity of the
 cylinder, and multiplying that velocity by the area of the hydraulic
 circuit.
 The actual fluid flow to each of the hydraulic circuits is converted into
 an actual percentage of fluid flow by dividing each of the actual fluid
 flows by the sum of all the fluid flows, as shown at block 76. The actual
 percentage of fluid flow for each hydraulic circuit is then compared with
 its corresponding desired percentage of fluid flow, as shown at
 conditional block 78, to determine if there is a difference. If so,
 controller 36 then increases or decreases the amount of current applied to
 the solenoids 38 of the hydraulic circuits not receiving the desired
 percentage of fluid flow, as shown at block 80.
 Of course, various modifications of this invention would come within the
 scope of the invention. The main fundamental concept is to automate
 calibration of an electrohydraulic system to determine current to flow
 percentage mapping for each hydraulic circuit in the system and to
 determine a scaling factor, or handicap, to be applied to those circuits
 that only get a fraction of maximum fluid flow.
 INDUSTRIAL APPLICABILITY
 The present invention is advantageously applicable in controlling the flow
 of fluid in an electrohydraulic system 10 having only one hydraulic pump
 22 coupled to a plurality of cylinders 12 as in construction machinery.
 Each of the cylinders 12 have at least hydraulic circuits 16, 18
 associated therewith for receiving the fluid and moving the cylinder 12
 correspondingly. Each of the cylinders 12 has a valve 20 coupled to the
 associated hydraulic circuits 16, 18 and the pump 22. Furthermore, each of
 the valves 20 are coupled to a solenoid 38 that moves the valve 20 to
 allow for fluid flow into one of the hydraulic circuits 16, 18. The
 following description is only for the purposes of illustration and is not
 intended to limit the present invention as such. It will be recognizable,
 by those skilled in the art, that the present invention is suitable for a
 plurality of other applications.
 The present invention begins by calibrating the electrohydraulic system 10
 to determine scaling factors for each of the hydraulic circuits 16, 18 in
 each of the cylinders 12. The scaling factors are applied to input
 commands to the hydraulic circuits 16, 18 to compensate for the hydraulic
 circuit 16, 18 receiving only a fraction of maximum fluid flow. The
 scaling factor is determined by stepping through a current range of the
 solenoid 38 and measuring steady state velocity of the cylinder 12 as it
 moves in response to application of the currents. Maps of current to fluid
 flows and percentages of fluid flows are then generated for each of the
 hydraulic circuits 16, 18. The scaling factor for each hydraulic circuit
 is then determined by comparing the maximum fluid flow of the hydraulic
 circuit with the maximum fluid flow of all of the hydraulic circuits.
 Then, in operation, the controller 36 determines an input command to be
 applied to each of the cylinders 12 via the solenoid 38 to achieve a
 desired movement velocity. The desired movement velocity is determined
 based on the amount of movement applied to the input devices 34. The
 desired velocity is converted into a relative velocity to account for the
 single pump 22 pumping fluid to all of the hydraulic circuits 16, 18.
 After determining the desired relative velocity, a desired percentage of
 fluid flow to each hydraulic circuit 16, 18 is determined based on the
 area of the hydraulic circuits 16, 18. Controller 36 then applies a
 current to the solenoids according to the previously determined mappings
 to obtain the desired percentage of fluid flow to each of the hydraulic
 circuits 16, 18.
 Closed loop control can be achieved by determining the actual flow of fluid
 to each of the hydraulic circuits 16, 18 via the position of the cylinder
 12 as sensed by position sensor 40. The actual flow is determined
 according to the position signal and the area of the hydraulic circuit 16,
 18. If the actual fluid flow does not agree with the desired fluid flow,
 controller 36 modifies the amount of current applied to the solenoids 38
 accordingly.
 Other aspects, objects and advantages of this invention can be obtained
 from a study of the drawings, the disclosure and the appended claims.