U.S. patent application number 14/004783 was filed with the patent office on 2014-03-06 for multiple function hydraulic system with a variable displacement pump and a hydrostatic pump-motor.
This patent application is currently assigned to HUSCO INTERNATIONAL, INC.. The applicant listed for this patent is Eric P. Hamkins, Joseph L. Pfaff. Invention is credited to Eric P. Hamkins, Joseph L. Pfaff.
Application Number | 20140060032 14/004783 |
Document ID | / |
Family ID | 45932503 |
Filed Date | 2014-03-06 |
United States Patent
Application |
20140060032 |
Kind Code |
A1 |
Pfaff; Joseph L. ; et
al. |
March 6, 2014 |
MULTIPLE FUNCTION HYDRAULIC SYSTEM WITH A VARIABLE DISPLACEMENT
PUMP AND A HYDROSTATIC PUMP-MOTOR
Abstract
A hydraulic system includes a first pump and a plurality of
valves that control fluid flow from the first pump to several
actuators. Variable source orifices in the control valves are
connected in parallel between the first pump and a node, and
variable bypass orifices in the control valves are connected in
series between the node and a tank. Pressure at the node controls
displacement of the first pump. Each control valve also has a
metering orifice for varying fluid flow between the node and one of
the actuators. A hydrostatic pump-motor, coupled between two ports
of a given actuator, is driven in a motoring mode by fluid exiting
one of those ports. In a pumping mode, the hydrostatic pump-motor
forces lower pressure fluid exhausting from one port into the other
port of the given actuator.
Inventors: |
Pfaff; Joseph L.;
(Wauwatosa, WI) ; Hamkins; Eric P.; (Waukesha,
WI) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Pfaff; Joseph L.
Hamkins; Eric P. |
Wauwatosa
Waukesha |
WI
WI |
US
US |
|
|
Assignee: |
HUSCO INTERNATIONAL, INC.
Waukesha
WI
|
Family ID: |
45932503 |
Appl. No.: |
14/004783 |
Filed: |
March 15, 2012 |
PCT Filed: |
March 15, 2012 |
PCT NO: |
PCT/US12/29175 |
371 Date: |
November 21, 2013 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
61452885 |
Mar 15, 2011 |
|
|
|
Current U.S.
Class: |
60/422 |
Current CPC
Class: |
F15B 7/003 20130101;
F15B 2211/654 20130101; F15B 2211/20546 20130101; E02F 9/2285
20130101; E02F 9/2292 20130101; F15B 2211/3116 20130101; E02F
9/2235 20130101; F15B 2211/6052 20130101; F15B 2211/253 20130101;
E02F 9/2242 20130101; F15B 11/17 20130101; E02F 9/2282 20130101;
E02F 9/2296 20130101; F15B 11/165 20130101 |
Class at
Publication: |
60/422 |
International
Class: |
F15B 7/00 20060101
F15B007/00 |
Claims
1. A hydraulic system for a machine comprising: a plurality of
hydraulic actuators, including a given hydraulic actuator that has
a cylinder and piston arrangement with a first port and a second
port; a first pump for furnishing fluid for operating the plurality
of hydraulic actuators; a tank for receiving fluid from the
plurality of hydraulic actuators; a plurality of control valves,
each of which selectively controls fluid flow between an associated
one of the plurality of hydraulic actuators and both the first pump
and the tank, the plurality of control valves including a given
control valve having a first workport connected to the first port
of the given hydraulic actuator and a second workport connected to
the second port; and a hydrostatic pump-motor operatively connected
to convey fluid between the first and second ports of the given
hydraulic actuator, and having a motoring mode in which the
hydrostatic pump-motor is driven by fluid flowing out of one of the
first and second ports at a higher pressure than fluid flowing into
the other one of the first and second ports, and having a pumping
mode in which fluid is pumped by the hydrostatic pump-motor when
fluid flowing out of one of the first and second ports has a lower
pressure than is required for fluid to flow into the other one of
the first and second ports.
2. The hydraulic system as recited in claim 1 wherein the first
pump has a variable displacement.
3. The hydraulic system as recited in claim 1 wherein the
hydrostatic pump-motor has a variable displacement.
4. The hydraulic system as recited in claim 3 wherein the given
control valve is operated by a control signal; and the variable
displacement of the hydrostatic pump-motor is controlled by the
control signal.
5. The hydraulic system as recited in claim 1 wherein when the
hydrostatic pump-motor is in the motoring mode, the given control
valve conveys some of the fluid flowing out of the one of the first
and second ports to the tank.
6. The hydraulic system as recited in claim 1 wherein when the
hydrostatic pump-motor is in a pumping mode, the given control
valve conveys fluid from the first pump into the other one of the
first and second ports.
7. The hydraulic system as recited in claim 1 further comprising: a
flow summation node in fluid communication with a control port for
controlling displacement of the first pump; and wherein each of the
plurality of control valves comprises a metering orifice for
varying fluid flow between the flow summation node and the
associated hydraulic actuator, a variable flow source orifice, and
a variable bypass orifice, the variable flow source orifices of the
plurality of control valves being connected in parallel between the
first pump and the flow summation node, and the variable bypass
orifices of plurality of control valves being connected in series
to form a bypass passage through which fluid flows between the flow
summation node and the tank.
8. The hydraulic system as recited in claim 7 wherein in each of
the plurality of valves, the variable flow source orifice enlarges
as the metering orifice enlarges, and the variable flow source
orifice shrinks as the metering orifice shrinks.
9. The hydraulic system as recited in claim 7 wherein in each of
the plurality of valves, the variable bypass orifice shrinks as the
metering orifice enlarges, and the variable bypass orifice enlarges
as the metering orifice shrinks.
10. A hydraulic system for a machine comprising: a plurality of
hydraulic actuators, including a given hydraulic actuator that has
a cylinder and piston arrangement with a first port and a second
port; a first pump for furnishing fluid for operating the plurality
of hydraulic actuators; a tank for receiving fluid from the
plurality of hydraulic actuators; a plurality of control valves,
each of which selectively controls fluid flow between one of the
plurality of hydraulic actuators and both the first pump and the
tank, the plurality of control valves including a given control
valve having a first workport connected to the first port of the
given hydraulic actuator and a second workport connected to the
second port; and a hydrostatic pump-motor operatively connected to
convey fluid between the first and second ports of the given
hydraulic actuator, and having a motoring mode in which the
hydrostatic pump-motor is driven by fluid flowing out of one of the
first and second ports at a higher pressure than fluid flowing into
the other one of the first and second ports.
11. The hydraulic system as recited in claim 10 wherein when the
hydrostatic pump-motor is in the motoring mode, the given control
valve conveys some of the fluid flowing out of the one of the first
and second ports to tank.
12. The hydraulic system as recited in claim 10 wherein the
hydrostatic pump-motor further has a pumping mode in which fluid is
pumped by the hydrostatic pump-motor when fluid flowing out of one
of the first and second ports has a lower pressure than is required
for fluid to flow into the other one of the first and second
ports.
13. The hydraulic system as recited in claim 12 wherein when the
hydrostatic pump-motor is in a pumping mode, the given control
valve conveys fluid from the first pump into the other one of the
first and second ports.
14. The hydraulic system as recited in claim 10 wherein the
hydrostatic pump-motor is a variable displacement type.
15. The hydraulic system as recited in claim 14 wherein the given
control valve is operated by a control signal; and displacement of
the hydrostatic pump-motor is controlled by the control signal.
16. The hydraulic system as recited in claim 10 further comprising
a pilot operated load check valve that opens in response to a pilot
signal to allow fluid flow from the given hydraulic actuator to the
second workport and otherwise allows fluid flow only from the
second workport to the given hydraulic actuator.
17. The hydraulic system as recited in claim 10 further comprising:
a flow summation node in fluid communication with a control port
for controlling displacement of the first pump; and wherein each of
the plurality of control valves comprises a metering orifice, a
variable flow source orifice, and a variable bypass orifice,
wherein the metering orifice varies fluid flow between the flow
summation node and the associated hydraulic actuator, the variable
flow source orifices of the plurality of control valves being
connected in parallel between the first pump and the flow summation
node, and the variable bypass orifices of plurality of control
valves being connected in series to form a bypass passage through
which fluid flows between the flow summation node and the tank.
18. The hydraulic system as recited in claim 17 wherein in each of
the plurality of valves, the variable flow source orifice enlarges
as the metering orifice enlarges, and the variable flow source
orifice shrinks as the metering orifice shrinks.
19. The hydraulic system as recited in claim 17 wherein in each of
the plurality of valves, the variable bypass orifice shrinks as the
metering orifice enlarges, and the variable bypass orifice enlarges
as the metering orifice shrinks.
20. A hydraulic system for a machine comprising: a plurality of
hydraulic actuators, including a given hydraulic actuator that has
a cylinder and piston arrangement with a first port and a second
port; a first pump for furnishing fluid for operating the plurality
of hydraulic actuators; a tank for receiving fluid from the
plurality of hydraulic actuators; a plurality of control valves,
each of which selectively controls fluid flow between one of the
plurality of hydraulic actuators and both the first pump and the
tank, the plurality of control valves including a given control
valve having a first workport connected to the first port of the
given hydraulic actuator and a second workport connected to the
second port; and a hydrostatic pump-motor operatively connected to
convey fluid between the first and second ports of the given
hydraulic actuator, and having a pumping mode which pumps fluid
when fluid flowing out of one of the first and second ports has a
lower pressure than is required for fluid to flow into the other
one of the first and second ports.
21. The hydraulic system as recited in claim 20 wherein when the
hydrostatic pump-motor is in the pumping mode, the given control
valve conveys fluid from the first pump into the other one of the
first and second ports.
22. The hydraulic system as recited in claim 20 wherein the
hydrostatic pump-motor further has motoring mode in which the
hydrostatic pump-motor is driven by fluid flowing out of one of the
first and second ports at a higher pressure than fluid flowing into
the other one of the first and second ports.
23. The hydraulic system as recited in claim 22 wherein when the
hydrostatic pump-motor is in the motoring mode, the given control
valve conveys some of the fluid flowing out of the one of the first
and second ports to tank.
24. The hydraulic system as recited in claim 20 wherein the
hydrostatic pump-motor is a variable displacement type.
25. The hydraulic system as recited in claim 24 wherein the given
control valve is operated by a control signal; and displacement of
the hydrostatic pump-motor is controlled by the control signal.
26. The hydraulic system as recited in claim 20 further comprising
a pilot operated load check valve that opens in response to a pilot
signal to allow fluid flow from the given hydraulic actuator to the
second workport and otherwise allows fluid flow only from the
second workport to the given hydraulic actuator.
27. The hydraulic system as recited in claim 20 further comprising:
a flow summation node in fluid communication with a control port
for controlling displacement of the first pump; and wherein each of
the plurality of control valves comprises a metering orifice for
varying fluid flow between the flow summation node and the
associated hydraulic actuator, a variable flow source orifice, and
a variable bypass orifice, the variable flow source orifices of the
plurality of control valves being connected in parallel between the
first pump and the flow summation node, and the variable bypass
orifices of plurality of control valves being connected in series
to form a bypass passage through which fluid flows between the flow
summation node and the tank.
28. The hydraulic system as recited in claim 27 wherein in each of
the plurality of valves, the variable flow source orifice enlarges
as the metering orifice enlarges, and the variable flow source
orifice shrinks as the metering orifice shrinks.
29. The hydraulic system as recited in claim 27 wherein in each of
the plurality of valves, the variable bypass orifice shrinks as the
metering orifice enlarges, and the variable bypass orifice enlarges
as the metering orifice shrinks.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application claims benefit of U.S. provisional patent
application no. 61/452,885 filed on Mar. 15, 2011.
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT
[0002] Not Applicable
BACKGROUND OF THE INVENTION
[0003] 1. Field of the Invention
[0004] The present invention relates to hydraulic systems having a
plurality of pumps and a plurality of independently controllable
hydraulic actuators; and more particularly to controlling the
plurality of pumps and allocating the resultant fluid flow to the
plurality of hydraulic actuators.
[0005] 2. Description of the Related Art
[0006] Hydraulic systems have at least one hydraulic pump that
supplies pressurized fluid which is feed through control valves to
drive several different hydraulic actuators. A hydraulic actuator
is a device, such as a cylinder-piston arrangement or a hydraulic
motor that converts the flow of hydraulic fluid into mechanical
motion.
[0007] Because loads of different magnitudes act on the various
hydraulic actuators, the hydraulic pressure required to operate
each actuator can vary greatly at any point in time. On an earth
excavator, for example, the hydraulic actuators that raise the boom
typically require a relatively high pressure as compared to other
actuators that curl the bucket or move the arm. Thus, when the
operator is raising the boom at the same time the arm or bucket are
also moving, a significant portion of the fluid flow from the pump
will go to the lower pressure hydraulic actuators. Without some
further compensation mechanism, this deprives the boom actuator of
the necessary fluid required to operate as commanded. To maintain
the proper flow sharing among all the actuators, the hydraulic
systems use complex throttling mechanisms that add a pressure drop
to the lower pressure functions and prevent them from consuming a
disproportionately large amount of the fluid flow at times when
multiple actuators are operating. Different equipment manufacturers
use different throttling mechanisms. Some of these mechanisms use
pressure compensators and a load sensing pump, while other ones use
pilot pressure signals from the operator controls to create
throttling losses for the low pressure functions. All these
throttling losses generate heat and add inefficiency to the
hydraulic system in order to enable the multifunction operation
commanded by a machine operator.
[0008] It is desirable to avoid these intrinsic losses in
efficiency and energy while still maintaining the multifunction
performance desired by the operator.
SUMMARY OF THE INVENTION
[0009] A hydraulic system for a machine includes a variable
displacement first pump that furnishes pressurized fluid into a
supply conduit in order to operate a plurality of hydraulic
actuators. A return conduit is provided to receive fluid flowing
back to a tank from the plurality of hydraulic actuators.
[0010] A control valve assembly has a plurality of control valves,
each associated with a different one of the plurality of hydraulic
actuators. Each control valve comprises a metering orifice for
varying fluid flow between the first pump and the associated
hydraulic actuator.
[0011] The hydraulic system further comprises a hydrostatic
pump-motor connected to convey fluid between two ports of a given
hydraulic actuator. In a motoring mode, the hydrostatic pump-motor
is driven by fluid flowing out of one of the actuator ports at a
higher pressure than fluid flowing into the other one of the ports.
The hydrostatic pump-motor operates in a pumping mode when fluid
flowing out of one port has a lower pressure than is required for
fluid enter the other port.
[0012] One embodiment of the present hydraulic system further
comprises a flow summation node that is in fluid communication with
a control port for controlling displacement of the first pump. The
metering orifice in each control valve varies fluid flow between
the summation node and the associated hydraulic actuator. Each
control valve further comprises a variable flow source orifice, and
a variable bypass orifice. For example, each control valve is
preferably configured so that as the metering orifice enlarges, the
variable flow source orifice also enlarges and the variable bypass
orifice shrinks. Inversely in that configuration, as the metering
orifice shrinks, the variable flow source orifice also shrinks and
the variable bypass orifice enlarges.
[0013] In the control valve assembly, the variable flow source
orifices of the plurality of control valves are connected in
parallel between the first pump and the flow summation node. The
variable bypass orifices of plurality of control valves are
connected in series to form a bypass passage through which fluid
flows between the flow summation node and the return conduit.
BRIEF DESCRIPTION OF THE DRAWINGS
[0014] FIG. 1 is a pictorial view of an excavator having a
hydraulic system;
[0015] FIG. 2 is a diagram of a first hydraulic system for the
excavator;
[0016] FIG. 3 is a schematic functional diagram of the components
of the first hydraulic system in FIG. 2 that control the
displacement of a primary pump;
[0017] FIG. 4 is a diagram of a second hydraulic system for the
excavator; and
[0018] FIG. 5 is a diagram of a third hydraulic system for the
excavator.
DETAILED DESCRIPTION OF THE INVENTION
[0019] The term "directly connected" as used herein means that the
associated components are connected together by a conduit without
any intervening element, such as a valve, an orifice or other
device, which restricts or controls the flow of fluid beyond the
inherent restriction of any conduit. If a component is described as
being "directly connected" between two points or elements, that
component is directly connected to each such point or element.
[0020] Although the present invention is being described in the
context of use on an earth excavator, it can be implemented on
other types of hydraulically operated equipment.
[0021] With initial reference to FIG. 1, an excavator 10 comprises
a cab 11 that can swing clockwise and counter-clockwise on a
crawler 16. A boom assembly 12, attached to the cab, is subdivided
into a boom 13, an arm 14, and a bucket 15 pivotally attached to
each other. A pair of hydraulic piston-cylinder assemblies 17, that
are mechanically and hydraulically connected in parallel, raise and
lower the boom 13 with respect to the cab 11. On a typical
excavator, the cylinder of these assemblies 17 is attached to the
cab 11 while the piston rod is attached to the boom 13, thus the
force of gravity acting on the boom tends to retract the piston rod
into the cylinder. Nevertheless, the connection of the
piston-cylinder assemblies could be such that gravity tends to
extend the piston rod from the cylinder. The arm 14, supported at
the remote end of the boom 13, can pivot forward and backward in
response to operation of another hydraulic piston-cylinder assembly
18. The bucket 15 pivots at the tip of the arm when driven by yet
another hydraulic piston-cylinder assembly 19. The bucket 15 can be
replaced with other work heads.
[0022] With additional reference to FIG. 2, a pair of left and
right bidirectional travel motors 20 and 22 independently drive the
tracks 24 to propel the excavator over the ground. A bidirectional
hydraulic swing motor 26 selectively rotates the cab 11 clockwise
and counterclockwise with respect to the crawler 16.
[0023] The hydraulic motors 20, 22 and 26 and the hydraulic
piston-cylinder assemblies 17-19 on the boom assembly 12 are
generically referred to as hydraulic actuators, which are devices
that convert hydraulic fluid flow into mechanical motion. A given
hydraulic system may include other types of hydraulic
actuators.
[0024] With particular reference to FIG. 2, a hydraulic system 30
has six hydraulic functions 31-36 although a greater or lesser
number of such functions may be used in other hydraulic systems
that practice the present invention. Specifically there are left
and right travel functions 31 and 32 and a swing function 33. The
boom assembly includes a boom function 34, an arm function 35, and
a bucket function 36, referred to as implement functions.
[0025] Each hydraulic function 31, 32, 33, 34, 35 and 36
respectively comprises a control valve 41, 42, 43, 44, 45 and 46
and the associated hydraulic actuator 20, 22, 26, 17, 18 and 19.
The six control valves 41-46 combine to form a control valve
assembly 40. The control valves may be physically separate or
combined in a single monolithic assembly. Control valves 41-46
govern the flow of fluid between the associated hydraulic actuator
and both a variable-displacement primary pump 50 and a tank 51. The
primary pump 50 furnishes pressurized fluid to a primary supply
conduit 58 and the fluid return to the tank 51 through a return
conduit 60. The primary supply conduit 58 and return conduit 60 or
110 extend to each of the control valves 41-46.
[0026] The primary pump 50 is of a type such that the output
pressure is equal to a pressure applied to a load sense port 39
plus a fixed predefined amount referred to as the "pump margin".
The primary pump 50 increases or decreases its displacement in
order to maintain the desired pressure. As an example, if the
difference between the outlet pressure and control input port
pressure is less than the pump margin, the pump will increase the
displacement. If the difference between the outlet pressure and
control input port pressure is greater than the pump margin, then
pump displacement is reduced. It is commonly known that flow
through an orifice can be represented as being proportional to the
flow area and the square root of differential pressure. Since this
pump control method provides a constant differential pressure of
"pump margin", the flow out of the primary pump 50 will be linearly
proportional to the flow area between the pump outlet and load
sense port 39. Alternatively, the primary pump 51 may be a type
that has a positive displacement, non-positive displacement,
electrohydraulically controlled displacement, or a load sense
controlled displacement.
[0027] When multiple functions are demanding fluid the pump may be
at a relatively high displacement that can overload the engine
driving the pump and potentially cause the engine to stall. This
condition is detected by the engine controller which responds by
providing an alert signal to a system controller 57 for the
hydraulic system, see FIG. 3. The system controller 57 responds by
operating the load sense power control valve 37 which opens by
proportional amount to reduce the pressure that is applied at in
the load sense port 39 to control the outlet pressure of the
primary pump 50. This action reduces the load on the engine and
prevents stalling.
[0028] The system controller 57, in addition to receiving input
signals from various sensors on the excavator, also receives
signals from an operator interface 59 in the cab 11. The system
controller responds by producing signals that operate the valves in
the first hydraulic system 30.
[0029] Referring again to FIG. 2, each control valve 41-46 is an
open-center, three-position valve, such as a spool type valve, for
example. Although in the exemplary hydraulic system 30, the control
valves 41-46 are indicated as being operated by a pilot pressure
operated, one or more of them could be operated by a solenoid or a
mechanical linkage.
[0030] The common features of all the control valves will be
described with respect to the first control valve 41, then the
features that are unique to the control valve in some of the
hydraulic functions will be described. The first control valve 41
has a supply port 62 that is connected to the primary supply
conduit 58 from the primary pump 50. A variable flow source orifice
64 within the control valve provides fluid communication between
the supply port 62 and a flow outlet 66. The flow outlet 66 of the
first control valve 41 is connected to a load sense conduit 67 by a
function flow limiter 63 comprising a fixed orifice in parallel
with a check valve. The second control valve has a similar function
flow limiter 63, whereas the flow outlets 66 of the third through
sixth control valves 44-46 are directly connected to the load sense
conduit 67. A flow summation node 74 is defined in the load sense
conduit 67. Thus, each variable flow source orifice 64 within a
control valve provides a separate variable fluid path between the
primary supply conduit 58 and the flow summation node 74.
[0031] The flow outlet 66 also is connected to a metering orifice
inlet 70, either directly as for the first and second control
valves 41 and 42 or by a conventional load check valve 68 as for
the other control valves 43-46. A variable metering orifice 75
within the first control valve 41 selectively connects the metering
orifice inlet 70 to one of two workports 76 and 78 depending upon
the direction that the first control valve is moved from the
center, neutral position. The two workports 76 and 78 connect to
different ports on the associated hydraulic actuator, such as
actuator 20 in the left travel function 31. The first control valve
41 is normally biased into the center position in which both
workports 76 and 78 are closed.
[0032] The first control valve 41 also has a bypass orifice 80 that
is directly connected between a bypass inlet 79 and a bypass outlet
81 of that control valve. The bypass orifices 80 for each of the
other control valves 42-46 are connected in series to provide fluid
communication between the summation node 74 and the return conduit
60. Specifically for the exemplary hydraulic system 30, the bypass
inlet 79 of the fifth control valve 45 is connected to the
summation node 74. The bypass outlet 81 of that control valve 45 is
directly connected to the bypass inlet 79 of the fifth control
valve 45 whose bypass outlet is directly connected to the bypass
inlet 79 of the fourth control valve 44 and so on through all the
hydraulic functions. The bypass outlet 81 of the first control
valve 41 is connected directly to the return conduit 60. Thus the
series of the bypass orifices 80 is connected between the summation
node 74 and the return conduit 60.
[0033] Before describing the operation of the first hydraulic
system with fluid from other pumps 82, 83 and 84, the control of
the variable displacement primary pump 50 will be described. FIG. 3
illustrates those components of the first hydraulic system 30 that
control the displacement of a primary pump 50. The variable flow
source orifices 64 and the bypass orifices 80 are arranged in more
functional groupings with those respective orifices shown outside
the corresponding control valve 41-46 in which they are actually
located. This functional diagram shows that the variable flow
source orifices 64.sub.41-64.sub.46, of all the control valves
41-46 are connected in parallel between the primary supply conduit
58 from the primary pump 50 and the flow summation node 74. This
parallel connection forms a variable flow section 65. The bypass
orifices 80.sub.41-80.sub.46 of all the control valves 41-46 are
connected in series between the flow summation node 74 and the
return conduit 60 to the tank 51 and form a bypass section 88 of
the hydraulic system 30. A subscript for an orifice's reference
number denotes the control valve 41-46 of which the corresponding
orifices is a part, e.g. bypass orifices 80.sub.41 is part of the
first control valve 41.
[0034] Assume initially that all the control valves 41-46 are in
the center position in which both their workports 76 and 78 are
closed off. In that state, the output from the primary pump 50,
applied to primary supply conduit 58, passes through the variable
flow source orifices 64.sub.41-64.sub.46, all of which are now
shrunk to relatively small flow areas. Therefore, a relatively
small amount of fluid flows from the primary pump 50 through the
variable flow section 65 to the summation node 74. At this time,
all the bypass orifices 80.sub.41-80.sub.46 in the bypass section
88 are enlarged to provide relatively large flow areas, thereby
allowing the fluid entering the summation node 74 to pass easily
into the return conduit 60. As a consequence, the pressure at the
fluid summation node 74 is at a relatively low level. That low
pressure level is transmitted through the load sense conduit 67 to
the load sense port 39 of the variable displacement primary pump
50. This results in a low outlet pressure at the primary pump.
[0035] Alternatively when a control valve 41-46 is in the center
position, its variable flow source orifice 64.sub.41-64.sub.46 can
be fully closed so that no fluid flows through that control valve
between the primary supply conduit 58 and the flow summation node
74. In this version of the pump control system, a separate small,
fixed orifice may be added to connect the primary supply conduit 58
to the flow summation node 74 in the variable flow section 65, so
that some flow from the primary supply conduit enters the flow
summation node when all the control valves are in the center
position.
[0036] Referring to FIGS. 2 and 3, operation of the pump control
technique will be described in respect of the left travel function
31 with the understanding that the other hydraulic functions 32-36
operate in the same manner. The opening movement of the first
control valve 41 in either direction from the center position
connects the metering orifice inlet 70 through the variable
metering orifice 75 to one of the workports 76 or 78, depending
upon the direction of that motion. Opening the first control valve
41 also connects the other workport 78 or 76 to the return conduit
60. At the same time, the variable flow source orifice 64.sub.41
enlarges by an amount related to the distance that the control
valve moves, thereby causing the pump to increase fluid flow from
the primary supply conduit 58 to the flow summation node 74 in
order to maintain the "pump margin," as previously described.
Simultaneously, the size of the bypass orifice 80.sub.41 shrinks,
causing pressure at the summation node 74 to increase. Thus as the
first control valve 41 opens a path through which fluid is supplied
to the left travel hydraulic actuator 20, the flow through the
variable flow section 65 into the summation node 74 increases,
while the restriction, created by bypass orifice 80.sub.41 to flow
occurring out of that node to the tank return conduit 60 also
increases thereby causing the pressure at the flow summation node
74 to increase.
[0037] When the flow summation node pressure is sufficiently great
to overcome the load force acting on the left travel hydraulic
actuator 20, fluid begins to flow through the metering orifice 75
in the first control valve 41 to drive the left travel hydraulic
actuator 20.
[0038] At the same time that the first control valve 41 is opening
one or more of the other control valves 42-46 also may be open.
Their respective variable flow source orifices 64.sub.42-64.sub.46
also will be conveying fluid from the primary supply conduit 58
into the flow summation node 74. Because all the variable flow
source orifices 64.sub.41-64.sub.46 are connected in parallel, the
same pressure differential is across each of those orifices. That
pressure differential and the cross sectional area of each flow
source orifice determines the amount of flow through that orifice.
The total flow into the flow summation node 74 is the aggregate of
the individual flows through each variable flow source orifice
64.sub.41-64.sub.46. As a result, the sum of the areas that each
variable flow source orifice is open determines the aggregate flow
into the flow summation node 74 and thus controls the output flow
from the variable displacement primary pump 50. The respective flow
area of the metering orifice 75 in each control valve 41-46 and the
respective load forces on actuators 17, 18, 19, 20, 22 and 26
determine the amount of flow each actuator receives from the flow
summation node 74.
[0039] When the left travel hydraulic actuator 20 moves the
excavator to the desired position, the first control valve 41 is
returned to the center position by whatever apparatus controls that
valve. In the center position, the two workports 76 and 78 are
closed again cutting off fluid flow from the flow summation node 74
to the left travel hydraulic actuator 20. In addition, the
associated variable flow source orifice 64.sub.41 shrinks to a
relatively small size which reduces the flow from the primary
supply conduit 58 to the flow summation node 74. Returning the
first control valve 41 to the center position also enlarges the
size of its bypass orifice 80.sub.41. Now if the other control
valves 41-46 also are in the center position, all their bypass
orifices 80.sub.41-80.sub.46 are relatively large, thereby
relieving the flow summation node pressure into the return conduit
60.
[0040] Referring again to FIG. 2, a principal feature of the first
hydraulic system 30 is the additional incorporation of one or more
fixed displacement pumps 82, 83, and 84. Specifically, the left
travel hydraulic function 31 has a first fixed displacement pump 82
that conveys fluid from the tank 51 into a first secondary supply
conduit 91 which is connected to a secondary inlet port 92 on the
first control valve 41. In the center or neutral, position of the
first control valve 41, the secondary inlet port 92 is connected to
a secondary outlet port 93 that in turn is coupled by a check valve
94 to a shared supply conduit 95.
[0041] The right travel function 32 includes a similar second fixed
displacement pump 83 that furnishes pressurized hydraulic fluid to
a second secondary supply conduit 96 connected to a second
secondary inlet port 97 of the second control valve 42. That
control valve has a second secondary outlet port 98 that is coupled
by a check valve 99 to the shared supply conduit 95. Note that the
two check valves 94 and 99 allow fluid to flow only from the
respective secondary outlet port 93 or 98 into the shared supply
conduit 95 and do not allow fluid to flow in the opposite
direction.
[0042] The shared supply conduit 95 is connected directly to a
third secondary inlet port 100 of the third control valve 43 for
the swing function 33. As will be described, this connection
provides a secondary source of hydraulic fluid to that third
control valve 43 for use in operating the swing hydraulic actuator
26.
[0043] The boom function 34 includes a third fixed displacement
pump 84 having an outlet that is connected to a tertiary supply
conduit 102 that leads to a tertiary inlet port 104 of the fourth
control valve 44. The fourth control valve 44 has a tertiary outlet
port 106 that is directly connected to a secondary tank return
conduit 110. In the center, or neutral, position of the fourth
control valve the tertiary inlet port 104 is coupled directly to
the tertiary outlet port 106 thereby allowing the outlet flow from
the third fixed displacement pump 84 to flow directly to the
secondary tank return conduit 110.
[0044] The displacement of the third fixed displacement pump 84 for
the boom function 34 is selected to provide a predefined amount
(e.g., 25 percent) of the total displacement of the pumps on a
conventional hydraulic system for an excavator. For example, if a
conventional excavator has two 100 cc pumps, then the third fixed
displacement pump 84 would have a fixed displacement of 50 cc.
Thus, when the fourth control valve 44 is moved into one of the
open positions, fluid from the third fixed displacement pump 84 is
always furnished to the boom hydraulic actuators 17.
[0045] The fixed displacement first and second fixed displacement
pumps 82 and 83 have lower maximum output flow and pressure than
the maximum flow and pressure available from the variable
displacement primary pump 50 and are associated with hydraulic
functions that normally require a significantly lower output
pressure than that provided by the primary pump 50. For example,
the variable displacement primary pump 50 may have a maximum output
flow of 150 cc and the maximum output flow from each of the first
and second fixed displacement pumps 82 and 83 may be 25 cc. The
third fixed displacement pump 84 for the boom function may have a
50 cc maximum output flow, for example.
[0046] When the boom 13 is raising, all the fluid output from the
third fixed displacement pump 84 is directed to the head chambers
of the boom hydraulic actuators 17. Additional fluid from the
primary supply conduit 58 also will be directed through the fourth
control valve 44 to the head chambers of the boom hydraulic
actuators 17. As noted previously, because of the heavy weight of
the boom assembly 12, relatively high pressure is required to drive
the boom hydraulic actuators 17 and raise the boom. In contrast,
the arm and bucket functions 35 and 36 usually require
significantly lower pressure even under their greatest load
conditions. If other hydraulic functions consume all of the
available output from the variable displacement primary pump 50,
the boom 13 still will raise with the output from the third fixed
displacement pump 84, albeit that motion is slower than if fluid
also is available from the primary supply conduit 58. This use of a
dedicated third fixed displacement pump 84 for the boom function 34
precludes the need to incorporate throttling losses in the lower
pressure functions on the excavator.
[0047] On a conventional excavator, the travel functions are
typically given priority for the available fluid produced from the
shared pumps. This is because while travel motion is occurring, the
movement of the boom, arm, and bucket are usually incidental to the
travel. Thus on a previous excavator, the entire flow from one of
the pumps was directed exclusively to the travel function, while
the flow from another pump was utilized for the implement functions
(boom, arm and bucket). Flow that was not consumed by the implement
functions was routed back to the travel functions through a series
of check valves and orifices. This type of system typically
maintained one-half the machine's overall fluid displacement for
the travel function regardless of the flow requirements of the
implement functions. Although such a conventional circuit was
effective, it did not provide priority to the boom up function,
especially at low engine idle speeds, since the pressure allowed
for the implement functions was limited by a fixed orifice. This
previous hydraulic circuit also encountered controllability
problems during a transition from a mode that combined the output
from the two pumps to one in which one pump was used for travel and
the other for the implement functions. The present circuit with
separate first and second pumps for the travel function overcomes
these disadvantages.
[0048] The first and second fixed displacement pumps 82 and 83
provide flow priority to each of the left and right travel
functions 31 and 32, respectively, regardless of the flow
consumption of the other hydraulic functions 33-36. With reference
to FIG. 2, when the first control valve 41 is in one of the open
positions, the flow from the first fixed displacement pump 82 is
directed to the left travel hydraulic actuator 20. Any available
flow from the primary supply conduit 58 also is directed through an
internal orifice in the first control valve 41 to the left travel
hydraulic actuator 20. That internal orifice and the orifice in the
function flow limiter 63 restrict reverse flow of fluid produced by
the first fixed displacement pump 82 through the first control
valve and into the primary supply conduit 58, thereby robbing that
fluid from being used in driving the left travel actuator 20.
[0049] Note also that when one or both travel functions 31 and 32
is not consuming fluid from its respective first or second fixed
displacement pump 82 and 83, the associated control valve 41 or 42
conveys that fluid to the shared supply conduit 95 where it is
available for powering the swing function 33. If the swing function
is not operating, i.e. the third control valve 43 is in the center
position, the fluid from the shared supply conduit 95 flows through
that control valve into the tank return conduit 60. If, however,
the swing function 33 is operating, the fluid available in the
shared supply conduit 95 is applied along with fluid from the
primary supply conduit 58 to drive the swing hydraulic actuator 26.
This operation of the swing function is similar to how the travel
functions 31 and 32 combine fluid from the first and second fixed
displacement pumps 82 and 83 with fluid from the primary supply
conduit 58 to drive the travel hydraulic actuators. When both
travel functions 31 and 32 are being commanded 100%, the swing
function 33 can only draw fluid from the primary pump 50 and as a
consequence has its available flow limited. Because the swing
function is in parallel with the travel functions, the pressure
(and thus the torque) in the swing function also is limited.
[0050] Note that the load sense conduit 67 has a fixed orifice 69
between the connection points of that conduit for the two travel
functions 31 and 32 and the conduit connection points for the
remaining functions 33-36. With the first hydraulic system 30, the
variable displacement primary pump 50 has a higher flow capacity
than can be allowed into any one of the travel hydraulic actuators
20 or 22. As a consequence, limiting the application of the flow
from the primary pump 50 to the travel hydraulic actuators is
required. If the only function that is active is one of the two
travel functions 31 or 32, there is no travel over speed issue
since that travel function is in sole control of the primary pump
and thus receives 75% of its flow requirements from the primary
pump 50 and 25% of its flow from the first or second fixed
displacement pump 82 or 83, in the exemplary system. It should be
understood that other flow proportions can be employed in different
systems.
[0051] However, when one of the travel functions 31 or 32 is active
at the same time that at least one of the other functions 33-36 is
active, that other function could demand a greater amount of flow
from the primary pump 50, thereby increasing that pump's
displacement to a point where the travel function could be driven
into an undesirably high speed. To prevent this from occurring, the
load sense conduit 67 has a limiting orifice 69 separating the
conduit connections of the travel functions 31 and 32 from the
conduit connections for the other hydraulic functions 33-36. This
orifice 69 limits the degree to which those other functions can
command the displacement of the primary pump 50, while allowing the
active travel functions 31 and/or 32 to dominate that control.
Thus, the additional flow that is allowed into the travel functions
31 and 32 is limited to flow created through the flow limit orifice
69 with the pump margin acting across it. For instance, if the flow
limiting orifice 69 is sized to allow 25 lpm at 15 bar pressure
(the margin), the maximum flow beyond the single function flow into
the travel, is 25 lpm.
[0052] When both of the travel functions 31 and 32 are active, it
is necessary to prevent more than the maximum travel flow from
being directed to either travel hydraulic actuator 20 or 22. This
is accomplished by the function flow limiter 63 connected between
the flow outlet 66 of the control valve 41 and 42 and the load
sense conduit 67 in those travel functions 31 and 32. This function
flow limiter 63 comprises a fixed orifice in parallel with a check
valve that forces fluid flowing in the direction from the flow
outlet 66 to the load sense conduit 67 to pass through the fixed
orifice. As an example, if both of the left and right travel
functions 31 and 32 are commanded at 100% and then the left travel
actuator 20 stalls and cannot consume its commanded flow, the flow
will go through the orifice in the function flow limiter 63 for the
left travel function 31 and into the load sense conduit 67. That
flow continues through the load sense conduit 67 and the check
valve in the function flow limiter 63 of the right travel function
32. As a result, a pressure differential appears across the orifice
in the function flow limiter 63 of the left travel function 31 and
thus the extra flow that is received by the right travel function
32 from the left function is limited by this orifice with a
pressure differential equal to the margin pressure of the pump.
Under typical conditions, this additional flow will be sufficiently
small and does not result in over speed of the travel hydraulic
actuators 20 and 22.
[0053] FIG. 4 illustrates a second hydraulic system 200 that
embodies the present inventive concept. This hydraulic system 200
has a left travel function 201, and right travel function 202, a
boom function 203, a swing function 204, an arm function 205, and a
bucket function 206.
[0054] A variable displacement, primary pump 208 draws fluid from a
tank 210 and furnishes that fluid under pressure into a primary
supply conduit 209. The primary supply conduit 209 has a
two-position proportional control valve 207 that couples a first
section of that conduit, to which the left and right travel
functions 201 and 202 are connected, to a second section of the
primary supply conduit, to which the remaining hydraulic functions
203-206 are connected.
[0055] The second hydraulic system 200 has a fixed displacement
pump 220 which also draws fluid from the tank 210 and furnishes
that fluid under pressure through a supply check valve 222 to a
boom/arm selector valve 224. The boom/arm selector valve 224
directs the output flow from the fixed displacement pump 220 into
either a secondary supply conduit 228 or a bypass node 229. The
bypass node 229 is connected by a check valve 231 to the load sense
conduit 230. That check valve 231 prevents the flow from the fixed
displacement pump 220 from flowing into the load sense conduit and
thereby maintains the flow priority for the boom, swing, and arm
functions in that priority order. Another check valve 233 allows
fluid from the fixed displacement pump 220 that is not otherwise
consumed by certain hydraulic functions to flow into the primary
supply conduit 209 thus supplementing flow from the primary pump
208 for other hydraulic functions. This reduces the engine power
drawn by the primary pump 208.
[0056] Each hydraulic function 201, 201, 203, 204, 205 and 206
respectively comprises a control valve 211, 212, 213, 214, 215 and
216 and the associated hydraulic actuator 20, 22, 17, 26, 18 and
19. All the control valves 211-216 are connected to the primary
supply conduit 209 and to a return conduit 218 leading back to the
tank 210. The control valves 211-216 are open-center,
three-position types and may be a solenoid operated spool type
valve, for example. Each the control valve 211-216 has two open
states in which fluid from the primary supply conduit 209 is fed to
the associated hydraulic actuator 17-26 and fluid from the actuator
is returned through the valve to the tank return conduit 218.
Depending upon which open state is used, the hydraulic actuator is
driven in one of two directions.
[0057] The first and second control valves 211 and 212 have an
supply port 221 that is directly connected to the primary supply
conduit 209. An outlet port 223 of those control valves 211 and 212
is coupled by a function flow limiter 225 to a load sense conduit
230. The third, fifth and sixth control valves 213, 215 and 216
have similar supply ports 235 that are connected directly to the
primary supply conduit 209 and outlet ports 236 that are connected
directly to a load sense conduit section 238.
[0058] The fourth control valve 214 for the swing function 204 has
its supply port 237 coupled by a proportional flow limit valve 246
to the primary supply conduit 209 and has an outlet port 239 that
is connected directly to the load sense conduit section 238. Flow
limit valve 246 is pilot operated by the pressure at the outlet
port 239. The swing function 204 has a flow limiter that limits a
magnitude of the flow from the variable displacement pump from
exceeding the maximum flow rating for the swing hydraulic actuator
26. That flow limiter includes a flow valve 248 in series with a
fixed orifice 250 through which fluid being supplied to the swing
hydraulic actuator 26 travels. The flow valve 248 that is normally
open and is pilot operated by the pressure differential across the
orifice 250. Thus when the flow across the fixed orifice 250
exceeds a preset level, thereby producing a pressure drop of a
given magnitude, the flow valve 248 begins to close proportionally
thereby restricting the flow to the swing hydraulic actuator
26.
[0059] The load sense conduit section 238 is coupled by a fixed
summation orifice 242 to the load sense conduit 230 in which a flow
summation node 232 is defined. The load sense conduit 230 is
coupled by a fixed orifice 241 to the displacement control input
234 of the primary pump 208. When a control valve 211-216 is open,
fluid from the primary supply conduit 209 is applied to the flow
summation node 232 and the amount of that fluid application is
proportional to the degree to which the respective control is
open.
[0060] The control valves 211-216 also have bypass orifices 240
that are connected in series to form a bypass passage 226 between
the bypass node 229 and the tank return conduit 218. The bypass
passage 226 along with check valve 231 also provide a fluid path
between the summation node 232 and the return conduit 218. When all
the control valves 211-216 are in the closed, center position,
their bypass orifices 240 are enlarged to provide a relatively a
large flow path which permits fluid to pass easily from the bypass
node 229 to the return conduit 218. When a control valve 211-216
opens, its bypass orifice 240 shrinks restricting flow through the
bypass passage 226 which causes pressure at the summation node 232
to increase, thereby increasing the fluid supply pressure.
[0061] Note that there are sets of dual check valves 255, 260 and
262 at control valves 213, 215, and 214, respectively. When the
bypass passage 226 has a proper pressure therein, one of these
check valves can open to supply fluid from the bypass passage to
the respective control valve. The other check valve in the set
prevents that fluid from flowing backwards into the load sense
conduit 230 or into the primary supply conduit 209 in the open
state of the respective valve. These check valves 255, 260 and 262
allow fluid from both the primary supply conduit 209 and the fixed
displacement pump 220 to be supplied to a hydraulic function.
[0062] With continuing reference to FIG. 4, when either of the boom
up or the arm in motions is commanded, the flow from the fixed
displacement pump 220 is directed to the respective boom or arm
function 203 or 205. This is accomplished by activating boom/arm
selector valve 224 to proportionally direct the flow from the fixed
displacement pump 220 into the secondary supply conduit 228. This
prevents all of the fixed displacement pump flow from being
consumed by the travel functions 201 or 202 and more importantly
from being directed into the primary supply conduit 209 through the
check valve 233. The flow in the secondary supply conduit 228 is
directed into the bypass passage 226 through branch 253 at the boom
function 203. Note that check valve 254 in the bypass passage 226
blocks this flow from traveling back to the bypass node 229. Thus,
under all system conditions, if the boom function 203 is commanded,
the flow from the fixed displacement pump 220 is directed with
highest priority to maintain boom flow within the pressure limits
of that function. In this case where a boom up operation is
commanded, the bypass orifice 240 of the boom control valve 213
closes slightly, thereby forcing the fluid that has entered the
bypass passage 226 to flow through check valve 255 and the boom
control valve to the boom hydraulic actuators 17. This flow
supplements any flow that would otherwise be drawn from the primary
supply conduit 209.
[0063] Furthermore, during a digging operation of the excavator 10,
when the arm function 205 is active, the boom/arm selector valve
224 also sends flow from the fixed displacement pump 220 into the
secondary supply conduit 228. This flow also passes through the
branch 253 into the bypass passage 226 and from there through to
the arm function 205. Since the arm control valve 215 for that
function has a reduced bypass orifice 240, the bypass passage flow
is forced through a check valve 260 and the arm control valve to
power the arm hydraulic actuator 18. It is quite common during a
digging operation that the arm function 205 requires a higher
pressure than the bucket function 206. The second hydraulic system
200 maintains the higher pressure from the fixed displacement pump
220 for the arm function, while the variable displacement primary
pump 208 is allowed to run at a lower pressure as required by the
bucket function 206.
[0064] Note that between the boom function 203 and the swing
function 204, the bypass passage 226 is coupled through a check
valve 256 and a fixed orifice 258 to the primary supply conduit
209. This circuit branch allows fluid that is not consumed by the
arm function 205 to be directed into the primary supply conduit 209
from which it can be used by other hydraulic functions. Assuming
that the boom function 203 and the swing function 204 are
inoperative, when the arm function 205 is active, its bypass
orifice 240 in control valve 215 is at least partially closed
allowing fluid to flow into that function from the bypass passage
226 via the check valve 260. Any fluid that is not consumed by the
arm function 205 flows through the check valve 256 and the fixed
orifice 258. The fixed orifice 258 allows the pressure in the
bypass passage 226 to be maintained so that the arm function will
receive pressurized fluid.
[0065] When boom up, swing, and another lower pressure operation,
such as arm in or bucket curl, are being commanded, the swing
function 204 needs to maintain sufficient torque to accelerate
properly. Under this command scenario, the output flow from fixed
displacement pump 220 is directed to the boom function 203 via the
secondary supply conduit 228 and that function thereby operates at
the required pressure. The boom in or bucket curl operation are
powered from the primary pump 208 at a lower pressure. The swing
function 204, in order to accelerate, requires a higher pressure
than the variable displacement primary pump 208 is producing.
Therefore, the swing function 204 now is connected through the
check valve and orifice combination 264 that directs some of the
higher pressure flow in the secondary supply conduit 228 from the
boom function 203 to the swing function. The size of orifice at 264
is selected to limit the flow that is diverted from the boom
function.
[0066] Unlike the first hydraulic system 30, the second hydraulic
system 200 does not have separate pumps for the left and right
travel functions 201 and 202. Likewise, however, the variable
displacement primary pump 208 has a significantly higher flow
capacity than can be allowed into the travel hydraulic actuators 20
and 22 without an over speed condition occurring. When only one of
the travel functions 201 and 202 is operating, it is in control of
the primary pump 208 and thus receives the majority of its flow
requirement from that pump. The remainder of the flow requirement
is satisfied from the fixed displacement pump 220 via selector
valve 224 and check valve 233 supplying that fluid into the primary
supply conduit 209. When a single travel function is commanded
along with an implement function, such as the bucket function 206,
any additional flow to the travel functions 201 and 202 is limited
by the fixed summation orifice 242 in the load sense conduit 230.
As described previously with respect to the first hydraulic system
30, the same type of flow limiting occurs when both travel
functions are active.
[0067] Because the travel functions 201 and 202 do not have
separate fixed displacement pumps, a throttling priority technique
is implemented in the second hydraulic system 200. In this
instance, the control valve 207 separates the primary supply
conduit 209 into a first section 270 to which only the travel
functions 201 and 202 are connected and into a second section 272
to which the other functions 203-206 are connected. When a travel
function is commanded, this control valve 207 transitions from an
open position to a restricted position to limit the amount of flow
allowed from the primary pump 208 to the non-travel functions
203-206. The control valve 207 closes proportionally to the highest
pressure produced in the actuators for the two travel functions 201
and 202. In addition, the fixed summation orifice 242 in the load
sense conduit 230 limits the amount of pump outlet flow commanded
by the travel functions 201 and 202 that is allowed to flow to the
implement functions 203, 205 and 206 during this mode of
operation.
[0068] To avoid high pressure flow losses across the cross port
relief valves 266 at the hydraulic actuator 26 of the swing
function, a flow limit valve 246 is located in the flow path
through the swing control valve 214 between the primary supply
conduit 209 and the load sense conduit section 238. When the
pressure in that load sense conduit section 238 rises above a
preset level that is a little higher or a little lower than the
cross port relief valve pressure threshold, the pilot operated
control valve implementing this flow limit valve 246 closes to
thereby limit the swing function's inlet flow from the primary pump
208. Note that the flow limit valve 246 may be placed on either the
supply conduit side or the load sense conduit side of the swing
control valve 214.
[0069] To improve productivity and match the pressure load of the
bucket function 206 and the boom function 203, a throttling loss is
added in the exhaust conduit of the bucket function between the
control valve 216 and the tank return conduit 218. This restriction
varies in proportion to the boom up command. In the second
hydraulic system 200, this restriction is implemented by a
proportional control valve 268 that is operated in response to the
magnitude of the boom command. Alternatively, such a restriction
could be implemented by a variable orifice on the boom spool
through which the oil exhausting from the bucket function
flows.
[0070] With reference to FIG. 5, a third hydraulic system 300 has
six hydraulic functions, specifically a left travel function 301, a
right travel function 302, a swing function 303, a boom function
304, an arm function 305, and a bucket function 306. The third
hydraulic system 300 is similar to the first hydraulic system 30,
except for the boom function 304. As a consequence, the common
system components and elements have been assigned identical
reference numerals in both FIGS. 2 and 5.
[0071] Instead of the dedicated pump 84, the boom function 304
incorporates a hydrostatic pump-motor 310 connected in parallel
with the boom control valve 316. All the pumps 50, 82, 83 and 310
are driven by the engine of the excavator 10. The hydrostatic
pump-motor 310 has a first port that is connected by a first check
valve 311 to the tank return conduit 60, wherein the first check
valve allows fluid to flow only in a direction from the tank return
conduit 60 to that pump port and to the rod chamber of the boom
hydraulic actuators 17. The first port of the hydrostatic
pump-motor 310 also is connected directly to a first workport 314
of the boom control valve 316. A second port of the hydrostatic
pump-motor 310 is directly connected via an actuator conduit 319 to
a second workport 315 of the boom control valve 316. A second check
valve 312 is connected to allow fluid to flow from the tank return
conduit 60 to flow to the second port of the hydrostatic pump-motor
310 and to the second workport 315. The first and second check
valves 311 and 312 provide an anti-cavitation function ensuring
that sufficient make-up fluid is provided to maintain the
hydrostatic pump-motor 310 filled with fluid. Those check valves
311 and 312 can be removed if there otherwise is sufficient make-up
fluid to maintain that filled condition. The first workport 314 is
connected to the rod chambers of the boom hydraulic actuators 17
and the second workport 315 is coupled by a load check valve 318 to
the head chambers of those hydraulic actuators. The load check
valve 318 is pilot operated into an open state by a pressure signal
when the boom control valve 316 is in the position in which fluid
is to drain from those head chambers.
[0072] The control valve 316 for the boom function 304 is operated
by pilot pressures applied to opposite ends of that control valve
and those pilot pressures also are applied to a port of a
displacement control device 317 for the hydrostatic pump-motor 310.
Alternatively, electrical actuators can be used to operate both the
control valve 316 and the displacement control of the hydrostatic
pump-motor 310.
[0073] During a boom raise operation, the pilot signal that
operates the control valve 316 also controls the displacement of
the hydrostatic pump-motor 310. The initial supply fluid flow to
the boom hydraulic function 304 is provided via the control valve
316 from the flow summation node 74 in the same manner as that
described with respect to the first hydraulic system 30. As the
boom function 304 continues to be commanded, the hydrostatic
pump-motor 310 functions in the pumping mode in which fluid exiting
the rod chambers of the hydraulic actuators 17 is forced into the
head chambers. In the pumping mode, the fluid exiting the ports for
the head chambers of the hydraulic actuators is at a lower pressure
than is required for fluid to enter the port for the rod actuator
chamber. Therefore the hydrostatic pump-motor 310 must function as
a pump to increase the pressure of the fluid flowing through that
device.
[0074] If one or more other hydraulic functions 301-303 or 305-306
is operating simultaneously at a lower pressure and consuming fluid
at a higher flow rate, there may be inadequate fluid available in
the primary supply conduit 58 for the boom hydraulic function 304.
When this condition occurs, the hydrostatic pump-motor 310 may be
the only source of fluid for the boom hydraulic function.
Nevertheless, priority for the boom operation is maintained with
this hydraulic circuitry.
[0075] When a boom lower operation is commanded, the same pilot
signal that operates the boom control valve 316 also controls the
displacement of the hydrostatic pump-motor 310. In this mode, the
initial exhaust flow from the head chambers of the boom hydraulic
actuators 17 is governed by the boom control valve 316, which
directs that flow directly into the tank return conduit 60, with no
recovery of the energy in that fluid. Note that at this time, the
load check valve 318 is forced open by the pilot pressure signal
operating the boom control valve 316. As the machine operator
commands greater boom motion and the pilot pressure increases, the
hydrostatic pump-motor 310 begins operating in the motoring mode
and consumes some of the fluid exhausting into the actuator conduit
319 from the head chambers of the boom actuators 17. The fluid that
flows through the hydrostatic pump-motor 310 is sent into the
expanding rod chambers of those actuators. This action causes the
hydrostatic pump-motor 310 to begin motoring, thereby applying
mechanical energy onto the drive shaft connected to the hydrostatic
pump-motor and recovering energy from the exhausting fluid. The
rate of fluid consumption by the hydrostatic pump-motor 310 is
related to the magnitude of the operator commands and the resulting
pilot pressure signal. Since the head chamber exhaust flow from the
hydraulic actuators 17 is greater than the flow that can be
consumed by the hydrostatic pump-motor 310, the remainder of that
fluid flow is diverted to the tank 51 through the boom control
valve 316.
[0076] During boom lowering when the bucket 15 is driven downward
into the ground and the boom assembly 12 becomes a powered load,
the hydrostatic pump-motor 310 no longer recovers energy. Instead
the hydrostatic pump-motor 310 delivers energy to its port that is
connected to the rod chambers of the boom hydraulic actuators 17
without a change in the pump-motor displacement. Thus a smooth
transition occurs from motoring to pumping.
[0077] The foregoing description was primarily directed to one or
more embodiments of the invention. Although some attention has been
given to various alternatives within the scope of the invention, it
is anticipated that one skilled in the art will likely realize
additional alternatives that are now apparent from disclosure of
embodiments of the invention. Accordingly, the scope of the
invention should be determined from the following claims and not
limited by the above disclosure.
* * * * *