U.S. patent application number 13/114670 was filed with the patent office on 2012-11-29 for pump system having open-loop torque control.
Invention is credited to Jeffrey L. Kuehn, Bryan E. NELSON, Lawrence J. Tognetti.
Application Number | 20120301325 13/114670 |
Document ID | / |
Family ID | 47218004 |
Filed Date | 2012-11-29 |
United States Patent
Application |
20120301325 |
Kind Code |
A1 |
NELSON; Bryan E. ; et
al. |
November 29, 2012 |
PUMP SYSTEM HAVING OPEN-LOOP TORQUE CONTROL
Abstract
A pump system is disclosed. The pump system may have a pump with
a displacement that is variable, and an actuator movable to adjust
the displacement of the pump. The pump system may also have an
electro-hydraulic valve fluidly connected to the actuator and
configured to control movement of the actuator, and a variable
resistor mechanically connected to at least one of the actuator and
the pump. The variable resistor may be adjustable by movement of
the at least one of the actuator and the pump to vary a current
passing through the electro-hydraulic valve.
Inventors: |
NELSON; Bryan E.; (Lacon,
IL) ; Tognetti; Lawrence J.; (Peoria, IL) ;
Kuehn; Jeffrey L.; (Metamora, IL) |
Family ID: |
47218004 |
Appl. No.: |
13/114670 |
Filed: |
May 24, 2011 |
Current U.S.
Class: |
417/53 ;
417/213 |
Current CPC
Class: |
F15B 2211/20553
20130101; E02F 9/2296 20130101; F15B 21/08 20130101; E02F 9/2232
20130101; F15B 2211/63 20130101 |
Class at
Publication: |
417/53 ;
417/213 |
International
Class: |
F04B 49/22 20060101
F04B049/22; F04B 53/10 20060101 F04B053/10 |
Claims
1. A pump system, comprising: a pump having a displacement that is
variable; an actuator movable to adjust the displacement of the
pump; an electro-hydraulic valve fluidly connected to the actuator
and configured to control movement of the actuator; and a variable
resistor mechanically connected to at least one of the actuator and
the pump, and being adjustable by movement of the at least one of
the actuator and the pump to vary a current passing through the
electro-hydraulic valve.
2. The pump system of claim 1, wherein the actuator is fluidly
connected at a first end to an output of the pump and at an
opposing second end to the electro-hydraulic valve.
3. The pump system of claim 2, wherein the electro-hydraulic valve
includes: a pilot-operated valve element movable from a first
position at which the second end of the actuator is fluidly
connected to the output of the pump, and a second position at which
the second end of the actuator is fluidly connected to a
low-pressure tank; and a solenoid-operated valve element operable
to control movement of the pilot-operated valve element between the
first and second positions.
4. The pump system of claim 3, further including: at least one
working actuator; an outlet passage extending from the pump to the
at least one working actuator; a control valve disposed within the
outlet passage and configured to regulate operation of the at least
one working actuator; a first pilot passage extending from the
outlet passage, at a first location upstream of the control valve,
to a first end of the pilot-operated valve element; a second pilot
passage extending from the outlet passage, at a second location
upstream of the control valve and downstream of the first location,
to a second end of the pilot-operated valve element; and a
restricted orifice disposed within the second pilot passage.
5. The pump system of claim 4, wherein the solenoid-operated valve
element is configured to selectively relieve pressure in the second
pilot passage by an amount corresponding to current passing through
the electro-hydraulic valve.
6. The pump system of claim 5, wherein the pilot-operated valve
element is spring-biased toward the second position.
7. The pump system of claim 6, further including at least one of
pressure relief or pressure limiting device fluidly connected to
the second end of the pilot-operated valve element.
8. The pump system of claim 1, further including a signal
conditioning device located between the variable resistor and the
electro-hydraulic valve.
9. The pump system of claim 1, further including a controller in
communication with the electro-hydraulic valve and configured to
issue an open-loop torque command to the electro-hydraulic
valve.
10. The pump of claim 3, further including: at least one working
actuator; an outlet passage extending from the pump to the at least
one working actuator; a control valve disposed within the outlet
passage and configured to regulate operation of the at least one
working actuator; a first pilot passage extending from the outlet
passage, at a first location upstream of the control valve, to a
first end of the pilot-operated valve element; a second pilot
passage extending from the outlet passage, at a second location
between the control valve and the at least one working actuator, to
a second end of the pilot-operated valve element; and a restricted
orifice disposed within the second pilot passage.
11. A pump system comprising: a pump having a displacement that is
variable; at least one working actuator configured to receive
pressurized fluid from the pump; an outlet passage extending from
the pump to the at least one working actuator; a control valve
disposed within the outlet passage and configured to regulate
operation of the at least one working actuator; a pump actuator
movable to adjust the displacement of the pump, and being fluidly
connected at a first end to the outlet passage; a pilot-operated
valve element movable from a first position at which a second end
of the pump actuator is fluidly connected to the outlet passage,
and a second position at which the second end of the pump actuator
is fluidly connected to a low-pressure tank; a first pilot passage
extending from the outlet passage, at a first location upstream of
the control valve, to a first end of the pilot-operated valve
element; a second pilot passage extending from the outlet passage,
at a second location upstream of the control valve and downstream
of the first location, to a second end of the pilot-operated valve
element; a spring configured to bias the pilot-operated valve
element toward the second position; a restricted orifice disposed
within the second pilot passage; a solenoid-operated valve element
operable to control movement of the pilot-operated valve element
between the first and second positions; a variable resistor
mechanically connected to at least one of the actuator and the pump
and being adjustable by movement of the at least one of the
actuator and the pump to vary a current passing through the
solenoid-operated valve element; and a controller in communication
with the solenoid-operated valve element and configured to issue an
open-loop torque command to the solenoid-operated valve
element.
12. A method of controlling a pump, comprising: operating the pump
to pressurize a fluid; generating an electronic signal indicative
to adjust a displacement of the pump; and hydro-mechanically
adjusting the displacement of the pump based on the signal, wherein
hydro-mechanically adjusting the displacement the pump also
simultaneously modifies a resistance of the electronic signal.
13. The method of claim 12, wherein hydro-mechanically adjusting
the displacement of the pump includes continuously connecting a
pump outlet pressure with a first end of a pump actuator, and
selectively connecting a second end of the pump actuator to the
pump outlet pressure or a low-pressure tank.
14. The method of claim 13, wherein selectively connecting a second
end of the pump actuator to the pump outlet pressure or a
low-pressure tank includes directing the electronic signal to move
a first valve element that hydraulically biases a second valve
element between a first position and a second position.
15. The method of claim 14, further including: directing
pressurized fluid from the pump through a control valve to a
working actuator; directing first and second flows of pressurized
pilot fluid from upstream of the control valve to first and second
ends of the second valve element, respectively; and restricting the
second flow of pressurized pilot fluid, wherein movement of the
first valve element results in a least a portion of the second flow
of pressurized pilot fluid being directed to the low-pressure tank
to reduce a pressure of the second flow of pressurized pilot
fluid.
16. The method of claim 15, wherein an amount of the second flow of
pressurized pilot fluid directed to the low-pressure tank
corresponds with an amount of current in the electronic signal
passing to the first valve element.
17. The method of claim 16, further including mechanically biasing
the second valve element toward the second position.
18. The method of claim 17, further including relieving a pressure
of the second flow of pressurized pilot fluid when a pressure of
the second flow of pressurized pilot fluid exceeds a threshold
pressure.
19. The method of claim 14, further including: directing
pressurized fluid from the pump through a control valve to a
working actuator; directing a first flow of pressurized pilot fluid
from upstream of the control valve to a first and end of the second
valve element; directing a second flow of pressurized pilot fluid
from downstream of the control valve and upstream of the working
actuator to a second end of the second valve element; and
restricting the second flow of pressurized pilot fluid, wherein
movement of the first valve element results in a least a portion of
the second flow of pressurized pilot fluid being directed to the
low-pressure tank to reduce a pressure of the second flow of
pressurized pilot fluid.
20. The method of claim 12, wherein generating an electronic signal
includes generating an open-loop torque command.
Description
TECHNICAL FIELD
[0001] The present disclosure relates generally to a pump system,
and more particularly, to a pump system having open-loop torque
control.
BACKGROUND
[0002] Hydraulic tool systems typically employ multiple actuators
provided with high-pressure fluid from a common pump. In order to
efficiently accommodate the different flow and/or pressure
requirements of the individual actuators, the pump of these systems
generally has a variable displacement. That is, based on the
individual and/or combined flow and pressure requirements of the
actuators, the displacement of the pump changes to meet demands of
the actuators while remaining within torque absorption limitations
placed on the pump by an associated engine.
[0003] Generally, one or both of the pump's displacement and
discharge pressure are measured by different sensors, and an
associated controller responsively commands a corresponding
displacement change to manage torque absorption. An exemplary pump
of this type is described in U.S. Pat. No. 5,515,829 that issued to
Wear et al. on May 14, 1996 ("the '829 patent").
[0004] Although the pump described above may be adequate for many
applications, it may also require valuable computing time from the
controller and multiple feedback loops to properly maintain a
desired torque absorption as displacement and pressure change. The
multiple feedback loops can affect a responsiveness of the pump and
possibly cause pump and/or engine instabilities. In addition, the
sensors utilized for control of the pump may add unnecessary cost
and complexity to the system.
[0005] The disclosed pump system is directed to overcoming one or
more of the problems set forth above and/or other problems of the
prior art.
SUMMARY
[0006] In one aspect, the present disclosure is directed to a pump
system. The pump system may include a pump with a displacement that
is variable, and an actuator movable to adjust the displacement of
the pump. The pump system may also include an electro-hydraulic
valve fluidly connected to the actuator and configured to control
movement of the actuator, and a variable resistor mechanically
connected to at least one of the actuator and the pump. The
variable resistor may be adjustable by movement of the at least one
of the actuator and the pump to vary a current passing through the
electro-hydraulic valve.
[0007] In another aspect, the present disclosure is directed to
method of controlling a pump. The method may include operating the
pump to pressurize a fluid, and generating an electronic signal
indicative of a command to adjust a displacement of the pump. The
method may also include hydro-mechanically adjusting the
displacement of the pump based on the signal. Hydro-mechanically
adjusting the displacement the pump also simultaneously modifies a
resistance of the signal.
BRIEF DESCRIPTION OF THE DRAWINGS
[0008] FIG. 1 is a pictorial illustration of an exemplary disclosed
machine;
[0009] FIG. 2 is a schematic illustration of an exemplary disclosed
pump system that may be utilized in conjunction with the machine of
FIG. 1; and
[0010] FIG. 3 is a schematic illustration of another exemplary
disclosed pump system that may be used in conjunction with the
machine of FIG. 1.
DETAILED DESCRIPTION
[0011] FIG. 1 illustrates an exemplary machine 10 performing a
particular function at a worksite 12. Machine 10 may embody a
stationary or mobile machine, with the particular function being
associated with an industry such as mining, construction, farming,
transportation, power generation, oil and gas, or another industry
known in the art. For example, machine 10 may be an earth moving
machine such as the excavator depicted in FIG. 1, in which the
particular function includes the removal of earthen material from
worksite 12 that alters the geography of worksite 12 to a desired
form. Machine 10 may alternatively embody a different earth moving
machine such as a motor grader or a wheel loader, or a non-earth
moving machine such as a passenger vehicle, a stationary generator
set, or a pumping mechanism.
[0012] Machine 10 may be equipped with multiple systems that
facilitate operation thereof at worksite 12, for example a tool
system 14, a drive system 16, and an engine system 18 that provides
power to tool system 14 and drive system 16. During the performance
of most tasks, power from engine system 18 may be split between
tool system 14 and drive system 16. That is, during machine travel
between excavation sites, a mechanical output of engine system 18
may be converted to a rotation of traction devices that propel
machine 10, in some examples by way of a hydraulic or
hydro-mechanical transmission (not shown). When parked at an
excavation site and actively moving material, the mechanical output
of engine system 18 may be converted to hydraulic power supplied to
one or more working actuators of tool system 14.
[0013] As illustrated in FIG. 2, engine system 18 may include a
heat engine 20, for example an internal combustion engine, that is
coupled with a pump system 24. Pump system 24 may include a
collection of components that are driven by engine 20 to
hydraulically power tool and/or drive systems 14,16. Specifically,
pump system 24 may include a low-pressure tank 26, and a pump 28
fluidly connected to tank 26 by way of an inlet passage 30 and to
systems 14, 16 by way of an outlet passage 32. Pump 28 may be
driven by engine 20 to draw in low-pressure fluid from tank 26 and
discharge the fluid at an elevated pressure to systems 14, 16. Pump
system 24 may also include a displacement actuator 34 associated
with pump 28 and movable to vary a displacement of pump 28, a
displacement control valve 36 operable to cause movement of
displacement actuator 34, and a controller 38 configured to
regulate operation of displacement control valve 36.
[0014] Pump 28 may be a swashplate-type pump and include multiple
piston bores (not shown), and pistons (not shown) held against a
tiltable swashplate 40. One piston may be slidably disposed within
each of the bores and biased into engagement with a driving surface
(not shown) of swashplate 40. The pistons may reciprocate within
the piston bores to produce a pumping action as swashplate 40
rotates relative to the pistons (swashplate 40 may rotate while the
pistons and associated bores remain stationary, or the pistons and
bores may collectively rotate while swashplate 40 remains
stationary). Swashplate 40 may be selectively tilted relative to a
longitudinal axis of the pistons to vary a displacement of the
pistons within their respective bores. Although shown in FIG. 2 as
producing only a unidirectional flow of pressurized fluid, it is
contemplated that pump 28 may alternatively be an over-center pump
or rotatable in opposing directions to produce flows of fluid in
two directions, if desired.
[0015] When swashplate 40 rotates relative to the pistons, the
angled driving surface of swashplate 40 may drive each piston
through a reciprocating motion within each bore. When the piston is
retracting from the bore, fluid may be allowed to enter the bore
from inlet passage 30. When the piston is moving into the
associated bore under the force of the driving surface of
swashplate 40, the piston may force the fluid at an elevated
pressure from the bore toward systems 14, 16 via outlet passage 32.
The angular setting of swashplate 40 relative to the pistons may
affect a discharge rate of the pressurized fluid and be adjustable
by displacement actuator 34.
[0016] Displacement actuator 34 may include components that
function to adjust the tilt angle of swashplate 40 and subsequently
the effective displacement volume of each piston/bore paring of
pump 28. Specifically, displacement actuator 34 may include one or
more control pistons 42 that directly or indirectly press against a
portion of swashplate 40 to urge swashplate 40 to tilt relative to
the axial direction of the pump's pistons. In the disclosed
embodiment, control piston 42 is a dual-acting piston that is
movable in response to a force imbalance caused by fluid pressure
acting on opposing sides of a piston member. In particular, control
piston 42 may be continuously connected at one end (e.g., at a
rod-end) 44 to outlet passage 32 via a first actuator passage 46,
and selectively connected at an opposing end (e.g., at a head-end)
48 to outlet passage 32 via a second actuator passage 50. The
connection location of second actuator passage 50 to outlet passage
32 may be downstream of the connection location of first actuator
passage 46. When fluid of a sufficient pressure is introduced into
end 48 of displacement actuator 34, displacement actuator 34 may be
caused to move swashplate 40 from a maximum displacement position
toward a minimum displacement position by an amount and/or at a
rate corresponding to the force imbalance across the piston member
of displacement actuator 34. It is contemplated that displacement
actuator 34 may alternatively include a spring-biased single-acting
piston, if desired.
[0017] Displacement control valve 36 may be associated with
displacement actuator 34 to control a flow of fluid from outlet
passage 32 through second actuator passage 50 into second end 48,
thereby controlling in which direction (i.e., which of a
displacement-increasing and a displacement-decreasing direction)
swashplate 40 of pump 28 is moved by displacement actuator 34.
Displacement control valve 36 may be a spring-biased,
electro-hydraulic control valve that is movable based on a command
from controller 38. In particular, displacement control valve 36
may include a first valve element 52 that is pilot-operated to
control fluid flows to and from displacement actuator 34, and a
second valve element 54 that is solenoid-operated to control
movement of first valve element 52 when energized by controller
38
[0018] First valve element 52 may be movable between a first
position at which second end 48 of displacement actuator 34
receives pressurized fluid via second actuator passage 50, and a
second position (shown in FIG. 2) at which fluid flow through
second actuator passage 50 into second end 48 is blocked and second
end 48 is instead connected to tank 26. A first pilot passage 56
may direct pilot fluid from outlet passage 32 to a first end 58 of
first valve element 52 to urge first valve element 52 toward the
first position, and a second pilot passage 60 may direct pilot
fluid from outlet passage 32 to a second end 62 of first valve
element 52 to urge first valve element 52 toward the second
position. First pilot passage 56 may connect to outlet passage 32
at a location upstream of the connection locations of second
actuator passage 50 and second pilot passage 60 with outlet passage
32, and downstream of the connection location of first actuator
passage 46 with outlet passage 32. Second pilot passage 60 may
connect to outlet passage 32 at a connection location downstream of
second actuator passage 50 and upstream of systems 14, 16. First
valve element 52 may be spring-biased toward the second position. A
restricted orifice 64 may be placed within second pilot passage 60
to create a pressure drop that facilitates control of first valve
element 52.
[0019] Second valve element 54 may be movable from a first position
at which second pilot passage 60 is pressurized by fluid from
outlet passage 32, toward a second position at which second pilot
passage 60 is fluidly connected to tank 26. Second valve element 54
may be selectively energized by controller 38 to move toward the
second position, and spring-biased toward the first position. When
second valve element 54 is in the second position and second pilot
passage 60 is fluidly connected to tank 26, a pressure drop may be
generated across restricted orifice 64 (i.e., a pressure within
second pilot passage 60 between restricted orifice 64 and first
valve element 52 may be reduced) that allows the pressurized fluid
within first pilot passage 56 to move first valve element 52 toward
the second position. Second valve element 54 may be infinitely
variable, and movable to any position between its first and second
positions, thereby affecting a corresponding variable movement of
first element 54 between its first and second position and,
subsequently, a movement of displacement actuator 34. It should be
noted that the magnitude of the electrical current passing through
the solenoid of second valve element 54 may correspond with the
position achieved by second valve element 54. In other words, a
particular current may be selectively applied to second valve
element 54 by controller 38 to cause second valve element 54 to
move to a desired position that results in movement of first valve
element 52 also to a desired position and a corresponding desired
velocity of displacement actuator 34 and tilt angle of swashplate
40.
[0020] One or more pressure relief or pressure limiting valves 66
may also be fluidly communicated with second pilot passage 60.
Pressure relief valve 66 may be spring-biased and movable in
response to a pressure of second pilot passage 60 to selectively
connect second pilot passage 60 with tank 26, thereby relieving or
limiting excessive fluid pressures.
[0021] Controller 38 may embody a single or multiple
microprocessors, field programmable gate arrays (FPGAs), digital
signal processors (DSPs), etc. that include a means for controlling
an operation of pump system 24. Numerous commercially available
microprocessors can be configured to perform the functions of
controller 38. It should be appreciated that controller 38 could
readily embody a microprocessor separate from that controlling
other machine-related functions, or that controller 38 could be
integral with a machine microprocessor and be capable of
controlling numerous machine functions and modes of operation. If
separate from the general machine microprocessor, controller 38 may
communicate with the general machine microprocessor via datalinks
or other methods. Various other known circuits may be associated
with controller 38, including power supply circuitry,
signal-conditioning circuitry, actuator driver circuitry (i.e.,
circuitry powering solenoids, motors, or piezo actuators), and
communication circuitry.
[0022] Controller 38 may be in communication with second valve
element 54 via an electrical circuit 68, and be configured to
energize second valve element 54 by selectively directing an
electrical current through circuit 68. The amount of current
directed through circuit 68 by controller 38 may correspond with a
desired amount of torque that should be absorbed by pump 28 (i.e.,
with a torque limit of pump 28), and result in a specific change in
the tilt angle of swashplate 40. The desired amount of torque may
be determined based on operating conditions of engine 20, as is
known in the art, such that engine stall does not occur and engine
20 functions in an efficient manner. Because the solenoid of second
valve element 54 may be a relatively constant-resistance device,
controller 38 may vary the current passing through circuit 68 and
thereby regulate motion of second valve element 54 (and the
subsequent motion of swashplate 40), by adjusting a voltage applied
to circuit 68.
[0023] As is known in the art, an amount of torque absorbed by a
pump is proportional to a product of the pump's displacement and a
pressure of fluid discharged from the pump (i.e.,
Torque.apprxeq.Displacement.times.Pressure). Accordingly, when
controller 38 adjusts a voltage applied to second valve element 54
and the angle of swashplate 40 responsively changes, the amount of
torque absorbed by pump 28 should likewise change in an immediate
step-wise manner. However, this step-wise change in displacement
may also have a longer term effect on the pressure of pump system
24, as the displacement change may cause fluid to be discharged
into pump system 24 at a faster or slower rate (depending on the
displacement change direction). The rising or lowering of system
pressure, if left unchecked, could cause an actual torque
absorption of pump 28 to deviate away from the desired torque
absorption amount after the change in swashplate angle has been
implemented. In conventional systems, sensory feedback is required
to provide information regarding displacement position and/or
discharge pressure to help ensure that the desired amount of torque
is being absorbed. In the disclosed system, however, a desired
torque absorption of pump 28 may be maintained without the use of
any such sensors. Instead of additional sensors, pump system 24 may
include a variable resistor 67 that functions to adjust a total
resistance of circuit 68 as swashplate 40 moves, thereby varying
the amount of current passing through the solenoid of second valve
element 54.
[0024] In the disclosed embodiment, variable resistor 67 may be an
electro-mechanical device having a sliding contact (not shown),
also known as a wiper, that functions as an adjustable voltage
divider. The wiper may be mechanically connected to one or both of
displacement actuator 34 or swashplate 40, and be configured to
vary a resistance of circuit 68 during displacement-adjusting
movements of the associated component(s). In other words, as
displacement actuator 34 and/or swashplate 40 moves to adjust a
displacement of pump 28 in response to a voltage change implemented
by controller 38, the wiper of variable resistor 67 may also
simultaneously move to adjust a resistance of circuit 68. This
adjustment of the resistance of circuit 68, for a given applied
voltage, may result in a change in the current passing through
second valve element 54 and the torque absorption of pump 28.
Accordingly, variable resistor 67 may function as a feedback
mechanism for pump system 24 such that a desired torque absorption
level of pump 28 may be maintained, even as the pressure of system
24 changes as a result of a swashplate angle change.
[0025] A signal conditioning device 74 may be connected to circuit
68 and configured to condition the current passing through the
solenoid of second valve element 54, if desired. Although shown as
being located between variable resistor 67 and the solenoid of
second valve element 54, it is contemplated that signal
conditioning device 74 could be positioned at any other location
along circuit 68, such as between controller 38 and second valve
element 54 or between variable resistor 67 and a ground 76, as
desired. Signal conditioning device 74 may include, for example,
additional resistors, capacitors, amplifiers, and other known
electronic components.
[0026] FIG. 3 illustrates another embodiment of pump system 24.
Similar to pump system 24 of FIG. 2, pump system 24 of FIG. 3
includes tank 26, pump 28, displacement control valve 36, and
controller 38. However, pump system 24 of FIG. 3 may be a
load-sense type of system. That is, the connection of second
actuator passage 50 with outlet passage 32 may be located between a
valve stack 70 (i.e., a stack of one or more control valves) and a
working actuator 72 of tool and/or drive systems 14, 16. In this
manner, a load on working actuator 72 may be sensed and used to
help control the motion of first valve element 52.
INDUSTRIAL APPLICABILITY
[0027] The disclosed pump system may be applicable to any machine
where precise control over torque absorption in a simplified manner
is desired. The disclosed pump system may provide for precise
control of torque absorption by utilizing a variable resistor to
vary current directed through a displacement control valve based on
changing pump displacement. The disclosed system may be simple, as
no sensor or costly feedback control loops are required. Operation
of pump system 24 will now be described.
[0028] During operation, engine 20 may drive pump 28 to rotate and
pressurize fluid. The pressurized fluid may be discharged from pump
28 into outlet passage 32 and directed into working actuators 72 of
tool and/or drive systems 14, 16. As the pressurized fluid passes
through working actuators 72, hydraulic power in the fluid may be
converted to mechanical power used to move machine 10.
[0029] The fluid discharge direction and displacement of pump 28
may be regulated, at least in part, based on a desired torque
absorption amount. Controller 38 may determine the desired torque
absorption amount in a conventional manner, and then generate an
open-loop torque command that results in application of a
corresponding voltage to the solenoid of second valve element 54.
Second valve element 54, in response to the applied voltage, may
move to a particular position between its first (i.e.,
flow-blocking) position and its second (i.e., flow-passing
position), thereby moving first valve element 52 to a particular
position between its first and second position. The movement of
first valve element 52 to the particular position may result in a
desired force imbalance across the piston member of displacement
actuator 34 that functions to change a tilt angle of swashplate 40
and resulting displacement of pump 28. This displacement change, in
conjunction with the instantaneous pressure of pump system 24, may
cause pump 28 to absorb the desired amount of torque.
[0030] After the displacement of pump 28 has been changed, however,
the pressure within pump system 24 may gradually change (i.e.,
increase or decrease based on the displacement change direction).
This changing pressure, if left unaccounted, may cause the actual
torque absorption of pump 28 to deviate from the desired torque
absorption amount. Accordingly, as the displacement of pump 28
changes, variable resistor 67 may adjust the resistance of circuit
68. This adjustment to the resistance of circuit 68 may function to
change the current flowing through the solenoid of second valve
element 54, thereby varying the tilt angle of swashplate 40 such
that a relatively constant torque absorption of pump 28 may be
maintained.
[0031] It will be apparent to those skilled in the art that various
modifications and variations can be made to the disclosed pump
system. Other embodiments will be apparent to those skilled in the
art from consideration of the specification and practice of the
disclosed pump system. It is intended that the specification and
examples be considered as exemplary only, with a true scope being
indicated by the following claims and their equivalents.
* * * * *