HYBRID HYDRAULIC SYSTEMS FOR INDUSTRIAL PROCESSES
Cross Reference To Related Applications
This application is being filed on 4 October 201 , as a PCT
International Patent application in the name of Eaton Corporation, a U.S. national
corporation, applicant for the designation of all countries except the U.S., and,
QingHui Yuan, a citizen of China and Ankur Ganguli, a citizen of India, applicants
for the designation of the U.S. only, and claims priority to U.S. Patent Application
Serial No. 61/393,556 filed on 15 October 2010, the disclosure of which is
incorporated herein by reference in its entirety.
Background
In some conventional hydraulic systems, a fixed displacement pump
supplies fluid to one or more load sources. The load requirements of the load
sources vary over the duty cycle. The fixed displacement pump is sized to
accommodate the maximum load required during the duty cycle. Accordingly, the
pump may be oversized for a significant portion of the duty cycle.
Summary
Some aspects of the present disclosure relate to hydraulic systems
having repeating duty cycles (i.e., work cycles). The hydraulic systems include at
least a variable displacement pump and at least one accumulator to supply fluid to a
load section of the system. A control system determines when the accumulator is
charged and discharged. In certain implementations, the system also includes a
fixed displacement pump coupled in series with the variable displacement pump.
A variety of additional inventive aspects will be set forth in the
description that follows. The inventive aspects can relate to individual features and
combinations of features. It is to be understood that both the foregoing general
description and the following detailed description are exemplary and explanatory
only and are not restrictive of the broad inventive concepts upon which the
embodiments disclosed herein are based.
Brief Description of the Drawings
FIG. 1 is a hydraulic circuit diagram showing a fixed displacement
pump that is configured to supply fluid flow to a load section of an example
hydraulic system in accordance with the principles of the present disclosure;
FIG. 2 is a hydraulic circuit diagram showing a variable displacement
pump that is configured to supply fluid flow to a load section of another example
hydraulic system in accordance with the principles of the present disclosure;
FIG. 3 is a hydraulic circuit diagram showing a variable displacement
pump arrangement that is configured to supply fluid flow along a supply line to a
load section of another example hydraulic system in accordance with the principles
of the present disclosure;
FIG. 4 is a chart showing which drive modes correspond with which
load requirements;
FIG. 5 is a flowchart illustrating an operational flow for an example
control process 400 by which drive circuit of FIG. 3 may be operated;
FIG. 6 is a circuit diagram showing fluid flow through the drive
circuit when the drive circuit is configured in the charge mode;
FIG. 7 is a circuit diagram showing fluid flow through the drive
circuit when the drive circuit is configured in the dump mode;
FIG. 8 is a circuit diagram showing fluid flow through the drive
circuit when the drive circuit is configured in the normal mode;
FIG. 9 is a circuit diagram showing fluid flow through the drive
circuit when the drive circuit is configured in the discharge mode;
FIG. 10 is a graph charting the fluid flow to the supply line Ps from
each pump over time in an example hydraulic system having an example load profile
over an example duty cycle;
FIG. 11 is a graph depicting the difference in peak power supplied by
the motor to operate the pumps between a hydraulic drive system without an
accumulator (triangle line) and a hydraulic drive system having an accumulator
(circle line); and
FIG. 12 is a flowchart illustrating an operational flow for an example
design and selection process by which the pump arrangement of FIG. 3 of a
hydraulic system is designed and programmed.
Detailed Description
FIG. 1 is a hydraulic circuit diagram showing a hydraulic drive
circuit 100 for powering operation of a load section 130 having a repeating work
cycle (e.g., an injection molding machine). The drive circuit 100 includes a fixed
displacement pump 110 that is configured to supply fluid flow to the load section
130 of an example hydraulic system. An electronic control unit (ECU) 116 manages
the VFD 114.
The pump 110 has an inlet 1 1 and an outlet 113. The inlet 111
connects to a reservoir 105 (i.e., a tank) and the outlet 113 connects to a flow control
valve 120 and a relief valve 125. A motor 12 (e.g., an electric motor) having a
variable frequency driver (VFD) 114 drives the pump 110 to draw fluid from the
reservoir 105 and provide variable fluid flow to the system. The relief valve 125
controls the pump outlet pressure (i.e., the hydraulic system pressure) by passing
extra flow to the reservoir 105. The flow control valve 120 controls the flow rate of
the hydraulic fluid provided to the actuators. In certain implementations, the flow
control valve 120 and relief valve 125 are proportional valves.
In the example shown, the load section 130 includes a first load
source 132, a second load source 134, and a third load source 136. In other
implementations, the hydraulic system may have a greater or lesser number of load
sources. In certain implementations, each load source 132, 134, 136 includes a
machine actuator (e.g., an injection machine, a clamp, a screw, etc.). Each of the
actuators 132, 134, 136 is connected to the flow control valve 120 through a valve
131, 133, 135, respectively. In the example shown, the first valve 131 and the
second valve 133 are three-position valves and the third valve 135 is a two- position
proportional valve. In other implementations, however, the valves 131, 133, 135
may have any desired number of states.
Each of the actuators 132, 134, 136 switches between an active state,
in which the actuator 132, 134, 136 requires fluid flow from the pump 110, and an
inactive state, in which the actuator 132, 134, 136 is isolated from the pump 110.
The valves 131, 133, 135 are used to open and close fluid communication with the
actuators 132, 134, 136. In some implementations, only one load source 132, 134,
136 is active at any one time. In other implementations, multiple actuators 132, 134,
136 may be active simultaneously. In certain implementations, the actuators 132,
134, 136 perform iterative tasks so that the load required by each actuator 132, 134,
136 varies in accordance with a duty cycle.
For example, in some implementations, the hydraulic system includes
an injection molding system. In some such systems, the first load source 132
includes a hydraulic cylinder powering a clamp, the second load source 134 includes
a hydraulic cylinder that axially moves an auger of an injector, and the third load
source 136 includes a hydraulic motor that rotates that auger of the injector. In other
implementations, however, each load source 132, 134, 136 may include any desired
type of hydraulic actuator. While the machine load section 130 is depicted as an
injection molding machine, it will be appreciated that aspects of the present
disclosure are applicable to any type of hydraulic powered machine. In particular,
aspects of the present disclosure are suited for hydraulic machines having repeating
work cycles where the hydraulic pressure load and hydraulic flow load demanded by
the machines over the work cycles vary according to predefined profiles.
The hydraulic drive circuit 100 has an architecture that enables power
generation and power consumption to be effectively matched, thereby reducing
throttling power losses. In some implementations, one or more accumulators 150
may be connected to the hydraulic drive circuit 100 upstream of the f ow control
valve 120. In other implementations, one or more accumulators 150 may be
connected to the hydraulic drive circuit 100 in parallel with the flow control valve
120. For example, disclosure of one such parallel architecture drive circuit may be
found in copending U.S. Application No. [having attorney docket no.
15720.0122USU1], filed herewith, and titled "Hydraulic Drive Circuit with Parallel
Architectured Accumulator," which claims the benefit of U.S. Provisional
Application No. 61/393,968, filed October 18, 2010, and titled "Parallel
Architectured Intelligent Accumulator (PAIA) for Energy Saving," the disclosures
of both of which are hereby incorporated herein by reference.
Generally, the VFD 114 drives the pump 110 to supply fluid having a
particular pressure and flow rate to the load section 130. During the times in the
duty cycle when the load is below a first predetermined threshold, a valve
arrangement 155 directs a portion of the pumped fluid to charge the accumulator
150. During the times in the duty cycle when the required load is above a second
predetermined threshold, the valve arrangement 155 discharges the accumulator 150
to direct additional fluid flow to the load section 130.
The accumulator 150 is isolated from the remainder of the hydraulic
system (e.g., by valve arrangement 155) when the load requirement is between the
first and second thresholds. The motor 112, VFD 114, and pump 110 operate in
regular form when the accumulator 150 is isolated. In certain implementations, the
first and second thresholds are set so that the accumulator 150 is isolated from the
remainder of the hydraulic system over a significant portion of the duty cycle. In
some implementations, the first and second predetermined thresholds are set
experimentally based on the duty cycle and load requirements of a particular system.
FIG. 2 is a hydraulic circuit diagram showing a hydraulic drive
circuit 200 for powering operation of a load section 230 having a repeating work
cycle (e.g., an injection molding machine). The drive circuit 200 includes a variable
displacement pump 210 that is configured to supply fluid flow to a load section 230
of another example hydraulic system. The pump 210 has an inlet 2 11 and an outlet
213. The variable displacement pump 210 draws fluid from a supply tank 205
through the inlet 211. An ECU 216 controls a proportional valve 214 to selectively
provide fluid flow from the outlet 213 to the system. The pump 210, which is driven
by motor (e.g., an electric motor) 212, varies the fluid flow based on a load sense
signal LS. A proportional valve 220 downstream from the proportional valve 14
connects the variable displacement pump 210 to the load section 230. A
proportional relief valve 225 allows excess fluid from the variable displacement
pump 210 to return to the tank 205.
In the example shown, the load section 230 includes a first load
source 232, a second load source 234, and a third load source 236. In other
implementations, the hydraulic system may have a greater or lesser number of load
sources. In certain implementations, each load source 232, 234, 236 includes a
machine actuator (e.g., an injection machine, a clamp, a screw, etc.). Each of the
actuators 232, 234, 236 is connected to the flow control valve 220 through a valve
231, 233, 235, respectively. In the example shown, the first valve 231 and the
second valve 233 are three-position valves and the third valve 235 is a two- position
proportional valve. In other implementations, however, the valves 231, 233, 235
may have any desired number of states.
Each of the actuators 232, 234, 236 switches between an active state,
in which the actuator 232, 234, 236 requires fluid flow from the pump 210, and an
inactive state, in which the actuator 232, 234, 236 is isolated from the pump 210.
The valves 23 1, 233, 235 are used to open and close fluid communication with the
actuators 232, 234, 236. In some implementations, only one load source 232, 234,
236 is active at any one time. In other implementations, multiple actuators 232, 234,
236 may be active simultaneously. In certain implementations, the actuators 232,
234, 236 perform iterative tasks so that the load required by each actuator 232, 234,
236 varies in accordance with a duty cycle.
For example, in some implementations, the hydraulic system includes
an injection molding system. In some such systems, the first load source 232
includes a hydraulic cylinder powering a clamp, the second load source 234 includes
a hydraulic cylinder that axially moves an auger of an injector, and the third load
source 236 includes a hydraulic motor that rotates that auger of the injector. In other
implementations, however, each load source 232, 234, 236 may include any desired
type of hydraulic actuator. While the machine load section 230 is depicted as an
injection molding machine, it will be appreciated that aspects of the present
disclosure are applicable to any type of hydraulic powered machine. In particular,
aspects of the present disclosure are suited for hydraulic machines having repeating
work cycles where the hydraulic pressure load and hydraulic flow load demanded by
the machines over the work cycles vary according to predefined profiles.
The hydraulic drive circuit 200 has an architecture that enables power
generation and power consumption to be effectively matched, thereby reducing
throttling power losses. An accumulator 250 is connected to the hydraulic drive
circuit 200 upstream of the proportional valve 220 and downstream of the
proportional relief valve 225. In some implementations, one or more accumulators
250 may be connected to the hydraulic drive circuit 200 upstream of the flow control
valve 220. In other implementations, one or more accumulators 250 may be
connected to the hydraulic drive circuit 200 in parallel with the flow control valve
220. For example, disclosure of one such parallel architecture drive circuit may be
found in copending U.S. Application No. , filed herewith, and titled "Hydraulic
Drive Circuit with Parallel Architectured Accumulator," which claims the benefit of
U.S. Provisional Application No. 61/393,968, filed October 18, 2010, and titled
"Parallel Architectured Intelligent Accumulator (PAIA) for Energy Saving," the
disclosures of both of which are incorporated by reference above.
Generally, the variable displacement pump 210 supplies an
appropriate fluid flow to the load section 230 of the hydraulic system 200 based on
the load sense control signal LS. During the times in the duty cycle when the
required load is below a first predetermined threshold, a valve arrangement 255
directs a portion of the pumped fluid to charge the accumulator 250. During the
times in the duty cycle when the required load is above a second predetermined
threshold, the valve arrangement 255 discharges the accumulator 250 to direct
additional fluid flow to the load section 230.
The accumulator 250 is isolated from the rest of the hydraulic system
by the valve arrangement 255 when the load requirement is between the first and
second thresholds. The variable displacement pump 210 operates in regular form
when the accumulator 250 is isolated. In certain implementations, the first and
second thresholds are set so that the accumulator 250 is isolated from the rest of the
hydraulic system 200 over a significant portion of the duty cycle. In some
implementations, the first and second predetermined thresholds are set
experimentally based on the duty cycle and load requirements of a particular system.
FIG. 3 is a hydraulic circuit diagram showing a hydraulic drive
circuit 300 for powering operation of a load section 330 having a repeating work
cycle (e.g., an injection molding machine). The drive circuit 300 includes a variable
displacement pump arrangement 301 that is configured to supply fluid flow along a
supply line Ps to a load section 330 of another example hydraulic system. In some
implementations, the load section 330 is the same as load section 130 of FIG. 1 or
load section 230 of FIG. 2. In other implementations, however, the load section 330
may include any desired number and/or type of load sources. The variable
displacement pump arrangement 300 draws the fluid from a reservoir 305 and
dumps excess fluid to the reservoir 305.
The variable pump arrangement 300 includes a motor (e.g., an
electric motor) 310 driving at least a first pump 312 and a second pump 3 4 via a
drive shaft 319. At least the second pump 314 is a bi-directional pump. In certain
implementations, the second pump 314 is a variable displacement pump. In the
example shown, the first pump 312 is a fixed displacement, single direction pump.
In other implementations, the first pump 31 may be a variable displacement pump.
In still other implementations, the pump arrangement 300 may include additional
pumps (e.g., fixed displacement pumps and/or variable displacement pumps). The
pumps 312, 314 of the pump arrangement 300 are connected in series to the supply
line Ps .
The motor 310 constantly drives the first pump 3 12 to draw fluid
from the tank 305 through a pump inlet 311 and to supply the fluid from a pump
outlet 313 to the load section 330. The bi-directional, variable displacement pump
314 has a first port 315 and a second port 317. The motor 310 also constantly drives
the variable displacement pump 314 along with the first pump 312. However, since
the variable displacement pump 314 is bi-directional, each of the ports 315, 317 may
alternately function as an inlet port and an outlet port.
An ECU 316 provides a control signal Ul that controls when the
variable displacement pump 314 directs fluid in a first direction and when the
second pump 314 directs fluid in a second direction. When the control signal Ul
causes the second pump 314 to direct fluid in the first direction, the second pump
314 directs fluid from the pump supply line Ps (i.e., fluid obtained from the tank 305
by the first pump 312), through the second port 317, through the first port 315 to a
feeder line F. When the control signal Ul causes the second pump 314 to direct
fluid in the second direction, however, the second pump 314 directs fluid from the
feeder line F, through the first port 315, through the second port 317, to the pump
supply line Ps .
A three-position directional valve 320 selectively couples the feeder
line F to the tank 305 and to an accumulator arrangement including at least one
accumulator 340. Certain types of directional valves 320 also will selectively isolate
the feeder line F from both the tank 305 and the accumulator arrangement. The
directional valve 320 includes a feeder port 321, an accumulator port 323, and a
reservoir port 325. In the example shown, the directional valve 320 is configured to
move between three positions. In a neutral (e.g., middle) position, the directional
valve 320 does not connect any of the ports 321, 323, 325. Accordingly, the valve
320 does not fluidly couple the feeder line F to either the accumulator arrangement
or the tank 305. When moved to the left, the directional valve 320 connects the
feeder port 321 to the accumulator port 323, thereby fluidly coupling the feeder line
F to the accumulator arrangement. When moved to the right, the directional valve
320 connects the feeder port 321 to the reservoir port 325, thereby fluidly coupling
the feeder line F to the reservoir 305.
In the example shown in FIG. 3, the accumulator arrangement
includes one accumulator 340. In other implementations, however, the accumulator
arrangement may include an array of accumulators as shown in copending U.S.
Application No. , filed herewith, and titled "Hydraulic Drive Circuit with
Parallel Architectured Accumulator," the disclosures of both of which are
incorporated by reference above. This application also discloses a parallel
architecture for the accumulator arrangement that may be utilized in the drive circuit
300.
In certain implementations, the directional valve 320 is moved by a
solenoid 322, which is controlled by a control signal U2 generated by an ECU 318.
In some implementations, the ECU 318 is the same as ECU 316. In other
implementations, two separate ECUs 316, 318 may be provided. In certain
implementations, the directional valve 320 also may be spring-biased in one or both
directions (see springs 324).
Generally, the first pump 312 and the second pump 314 cooperate to
provide fluid to the supply line Ps for use by the load section 330. The first pump
312 supplies a constant fluid flow from the tank 305 to the load section 330. The
second pump 314 directs fluid from the reservoir 305 to the power supply line Ps ,
directs fluid from the accumulator 340 to the power supply line Ps, directs some of
the fluid output from the first pump 3 2 to the accumulator 340, or directs fluid from
the pump 314 to the reservoir 305 depending on a drive mode of the drive circuit
300. The drive mode changes based on the load required at any given time in the
duty cycle.
FIG. 4 is a chart showing which drive modes correspond with which
load requirements. As shown, when the load section 130 has a low load requirement
(e.g., when the load requirement of the load section 130 is less than a first threshold
Tl), the ECU 316 provides a control signal Ul instructing the second pump 314 to
direct fluid in the first direction from the feeder line F to the supply line Ps. During
a low power requirement, the drive circuit 300 may be configured into either a
charge mode or a dump mode. When configured in the charge mode, the ECU 318
provides a control signal U2 instructing directional valve 320 to connect the feeder
line F to the accumulator 340. When configured in the dump mode, the ECU 318
provides a control signal U2 instructing directional valve 320 to connect the feeder
line F to the reservoir 305.
When the load section 130 has a normal load requirement (e.g., when
the load requirement of the load section 130 is more than a first threshold Tl and
less than a second threshold T2), the ECU 316 provides a control signal Ul
instructing the second pump 314 to direct fluid in the second direction from the
supply line Ps to the feeder line F. The ECU 318 provides a control signal U2
instructing directional valve 320 to connect the feeder line F to the reservoir 305.
Accordingly, the second pump 314 is effectively drawing fluid from the tank 305
and directing the fluid to the supply line Ps in addition to the fluid being supplied by
the first pump 312.
When the load section 130 has a high load requirement (e.g., when
the load requirement of the load section 130 is greater than the second threshold T2),
the ECU 316 provides a control signal Ul instructing the second pump 314 to direct
fluid in the second direction from the supply line Ps to the feeder line F. The ECU
318 provides a control signal U2 instructing directional valve 320 to connect the
feeder line F to the accumulator 340. Accordingly, the second pump 314 is
effectively drawing fluid from the accumulator 340 and directing the fluid to the
supply line Ps in addition to the fluid being supplied by the first pump 312.
FIG. 5 is a flowchart illustrating an operational flow for an example
control process 400 by which drive circuit 300 of FIG. 3 may be operated. In some
implementations, the example control process 400 is implemented by one or more
electronic control units (e.g., the ECU 316 and ECU 318 of FIG. 3). In other
implementations, the example control process 400 may be executed by any other
electronic processor. The example control process 400 begins at a start module 402,
performs any appropriate initialization procedures, and proceeds to an obtain
operation 404.
The obtain operation 404 determines the current load requirement for
the hydraulic system. In some implementations, the obtain operation 404 compares
a clock time to a predetermined duty cycle of the hydraulic system. In other
implementations, the obtain operation 404 receives a control signal indicating the
current status of the system relative to a predetermined duty cycle. In other
implementations, the obtain operation 404 measures fluid pressure and fluid flow
using one or more sensors within the hydraulic system.
A first determination module 406 determines whether or not the
current load requirement is less than a first predetermined threshold Tl . If the
current load requirement is less than the first predetermined threshold Tl, then a first
pump position operation 408 causes the second pump 314 to direct fluid in the first
direction. For example, the ECU 316 may send a control signal Ul to the second
pump 314 to cause the second pump 314 to direct fluid from the supply line Ps to the
feeder line F.
A second determination module 410 determines whether the
accumulator (e.g., accumulator 340 of FIG. 3) is filled to capacity. If the
accumulator 340 is not filled to capacity, then a first valve position operation 412
moves the directional valve 320 to connect the feeder line F to the accumulator 340
for charging. For example, the ECU 318 may direct a solenoid to 322 to move the
directional valve 320 of FIG. 3 towards the left. Accordingly, the second pump 314
draws fluid from the supply line Ps and pumps the fluid to the accumulator 340
through the feeder line F. The control process 400 performs any appropriate
completion procedures and ends at a stop module 422.
For example, FIG. 6 is a circuit diagram showing fluid flow through
the drive circuit 300 when the drive circuit 300 is configured in the charge mode. In
FIG. 6, the ECU 316 has instructed the second pump 314 to direct fluid from the
supply line Ps to the feeder line F. The ECU 3 8 has instructed the solenoid to 322
to move the directional valve 320 to connect the feeder line port 321 to the
accumulator port 325. Accordingly, fluid flows from the first pump 3 2 to the
supply line Ps. Some of the fluid from the first pump 312 is drawn by the second
pump 314 from the supply line Ps and directed to the accumulator 340. Since the
fluid is flowing in the first direction, the valve connection is shown in FIG. 6 to
extend from the feeder port 3 1 to the accumulator port 323. It will be understood,
however, that the connection is bidirectional based on the direction of the second
pump 314.
Referring back to FIG. 5, if the second determination module 410
determines that the accumulator 340 is filled to capacity, however, then a second
valve position operation 414 moves the directional valve 320 to connect the feeder
line F to the reservoir 305. For example, the ECU 318 may direct a solenoid to 322
to move the directional valve 320 of FIG. 3 towards the right. Accordingly, the
second pump 314 draws fluid from the supply line Ps and dumps the fluid to the
tank 305 through the feeder line F. The control process 400 performs any
appropriate completion procedures and ends at a stop module 424.
For example, FIG. 7 is a circuit diagram showing fluid flow through
the drive circuit 300 when the drive circuit 300 is configured in the dump mode. In
FIG. 7, the ECU 316 has instructed the second pump 314 to direct fluid from the
supply line Ps to the feeder line F. The ECU 318 has instructed the solenoid to 322
to move the directional valve 320 to connect the feeder port 321 to the reservoir port
325. Since the fluid is flowing in the first direction, the valve connection is shown
in FIG. 7 to extend from the feeder port 321 to the reservoir port 325. It will be
understood, however, that the connection is bidirectional based on the direction of
the second pump 314.
In the example drive system 300 shown in FIG. 7, fluid flows from
the first pump 312 to the supply line Ps. Some of the fluid from the first pump 312
is drawn by the second pump 314 from the supply line Ps and directed to the
reservoir 305. Alternatively, in other implementations, the ECU 316 may instruct
the second pump 314 to pump fluid in the second direction to draw fluid from the
reservoir 305. In still other implementations, the ECU 318 may move the valve 320
to isolate both the accumulator 340 and the reservoir 305 from the feeder line F.
Referring back to FIG. 5, if the first determination module 406
determines that the current load requirement exceeds the first threshold, however,
then a second pump position operation 416 causes the second pump 314 to direct
fluid in the second direction. For example, the ECU 316 may send a control signal
Ul to the second pump 314 to cause the second pump 314 to direct fluid from the
feeder line F to the supply line Ps.
A third determination module 418 determines whether or not the
current load requirement exceeds a second predetermined threshold T2. If the
current load requirement does not exceed the second threshold, then the second
valve position operation 414 moves the directional valve 320 to connect the feeder
line F to the tank 305. The second pump 314 draws fluid from the tank 305 through
the feeder line F and pushes the fluid into the supply line Ps. Accordingly, first and
second pumps 312, 314 cooperate to supply fluid from the reservoir 305 to the load
section 330. The control process 400 performs any appropriate completion
procedures and ends at a stop module 424.
For example, FIG. 8 is a circuit diagram showing fluid flow through
the drive circuit 300 when the drive circuit 300 is configured in the normal mode. In
FIG. 8, the ECU 316 has instructed the second pump 314 to direct fluid from the
feeder line F to the supply line Ps. The ECU 318 has instructed the solenoid to 322
to move the directional valve 320 to connect the feeder port 321 to the reservoir port
325. Accordingly, fluid flows from the first pump 312 and from the second pump
314 to the supply line Ps. The second pump 314 draws the fluid from the reservoir
305.
If the current load requirement does exceed the second threshold,
however, then the first valve position operation 412 moves the directional valve 320
to connect the feeder line F to the accumulator 340 for discharging. Accordingly,
the second pump 314 draws fluid from the accumulator 340 through the feeder line
F and pumps the fluid into the supply line Ps. The control process 400 performs any
appropriate completion procedures and ends at a stop module 422.
For example, FIG. 9 is a circuit diagram showing fluid flow through
the drive circuit 300 when the drive circuit 300 is configured in the discharge mode.
In FIG. 9, the ECU 316 has instructed the second pump 314 to direct fluid from the
feeder line F to the supply line Ps . The ECU 318 has instructed the solenoid to 322
to move the directional valve 320 to connect the feeder line port 321 to the
accumulator port 323. Accordingly, fluid flows from the first pump 312 and from
the second pump 314 to the supply line Ps. The second pump 314 draws the fluid
from the accumulator 340.
FIG. 10 is a graph 800 charting the fluid flow (L/m) to the supply line
Ps from each pump 312, 314 over time (s) in an example hydraulic system having an
example load profile over an example duty cycle. The graph values were produced
in a numerical simulation. Since the graph 800 is presented to portray general trends
in the fluid flow during the various drive modes, the raw numbers obtained from the
simulation are unimportant to this disclosure.
The graph 800 shows the fluid rate of the first pump 312 (circle line)
remaining constant throughout the duty cycle while the fluid output rate of the
second pump 314 (triangle line) varies over the duty cycle. In particular, the fluid
rate of the second pump 314 drops below zero (e.g., 0 mL/sec) during portions of the
duty cycle. As indicated in FIG. 8, the fluid rate of the second pump 314 is positive
when the second pump 314 is directing fluid from the feeder line F to the supply line
Ps (e.g., when the second pump 314 is drawing fluid from the reservoir 305 or the
accumulator 340). The fluid rate of the second pump 314 is negative when the
second pump 314 is directing fluid from the supply line Ps to the feeder line F (e.g.,
to charge the accumulator 340).
During times in the system duty cycle when the load requirements of
the load section 330 are below a first predetermined threshold, however, the ECU
316 provides a control signal Ul instructing the second pump 314 to direct fluid
from the supply line Ps to the feeder line F. Furthermore, the ECU 318 provides a
second control signal U2 instructing the directional valve 320 to connect the feeder
line F to the accumulator 340. Accordingly, the second pump 314 is effectively
directing at least a portion of the fluid from the supply line Ps, through the feeder
line F, to the accumulator 340 to charge the accumulator 340.
FIG. 11 is a graph 700 depicting the difference in peak power
supplied by the motor 310 to operate the pumps 312, 314 between a hydraulic drive
system without an accumulator (triangle line) and a hydraulic drive system having
an accumulator (circle line). The graph values were produced in a numerical
simulation for an example hydraulic system having an example load profile over an
example duty cycle. Since the graph 700 is presented to portray how the addition of
an accumulator 340 generally affects the power requirements for the motor 310, the
raw numbers obtained from the simulation are unimportant to this disclosure.
As can be seen, the electric motor outputs a first level of power PI to
operate the pumps 312, 314 of the drive circuit 300 if the accumulator 340 is not
utilized. The electric motor outputs a second level of power P2 to operate the pumps
312, 314 of the drive circuit 300 if the accumulator 340 is utilized. The second
power level P2 is less than the first power level PI (see arrow). Accordingly, when
the accumulator 340 is added to drive circuit 300, the size of the motor 310 may be
reduced in comparison to the motor 310 necessary when the drive circuit 300 does
not include the accumulator 340. Reducing the motor size has multiple benefits.
First, smaller motors tend to be less costly to purchase and maintain. Second,
reducing the range of power required to be output by the motor enables the motor to
operate within its ideal operating range more often, which increases the efficiency of
the motor.
FIG. 12 is a flowchart illustrating an operational flow for an example
design and selection process 500 by which the pump arrangement 300 of FIG. 3 of a
hydraulic system is designed and programmed. For example, in some
implementations, the selection process 500 may be used to program the ECUs 316,
318. In certain implementations, the selection process 500 may be used to size the
pumps 312, 314. The example selection process 500 begins at a start module 502,
performs any appropriate initialization procedures, and proceeds to an obtain
operation 504. The obtain operation 504 acquires a load profile for the hydraulic
system. The load profile shows the change in load requirements of a load section of
the hydraulic system over a duty cycle of the hydraulic system. For example, in
some implementations, the obtain operation 504 runs a numerical simulation of the
hydraulic system to map the power requirements.
A calculate operation 506 determines an average or median load
required during the duty cycle of the hydraulic system. A size operation 508
determines the size requirements for the pump arrangement 300 to meet the average
or median load requirements. For example, the size operation 508 determines an
appropriate size for the first pump 312 and an appropriate size for the second pump
314. In some implementations, the size operation 508 selects pumps 312, 314 sized
to provide more than the average load, but significantly less than the maximum
required load. In other implementations, the size operation 508 selects pumps 312,
314 sized to provide the average load.
A second obtain operation 510 acquires a flow profile for the
hydraulic system including the selected pumps 312, 314 over the duty cycle. The
flow profile is based on the utilization of the pumps 312, 314 as sized in operation
508 without any accumulator. For example, in some implementations, the second
obtain operation 510 runs a numerical simulation of the hydraulic system to map the
fluid flow from the pumps 312, 314 to the load section 330 of the hydraulic system.
A second calculate operation 512 determines an appropriate first
threshold power level Tl and an appropriate second threshold power level T2 based
on the power profile and the flow profile. A control unit of the hydraulic system is
set to charge the accumulator 340 when the load requirement drops below the first
threshold level and to discharge the accumulator 340 when the load requirement
rises above the second threshold level. The control unit isolates the accumulator 340
when the load requirement is between the first and second thresholds. The first
threshold Tl is set at a value falling below the lowest power peak that would be
required of the VDP pump system. The second threshold T2 is set at a value falling
below the highest power peaks that would be required of the VDP pump system, but
well above the lower peaks. In certain implementations, the threshold levels Tl, T2
are set so that the power requirements of the hydraulic system fall within the normal
load range during a majority of the duty cycle.
The flow constraints of the system (e.g., the first and second
thresholds Tl, T2) are assessed by test operation 514. For example, the test
operation 514 may run another numerical simulation for the hydraulic system over
the duty cycle. In the numerical simulation run by test operation 514, the hydraulic
system includes both the pumps 312, 314 and the accumulator 340. The
accumulator 340 in the numerical simulation is charged, isolated, and discharged
based on the first and second threshold levels.
A determination module 516 checks the results of the numerical
simulation performed by the test operation 514 against one or more flow constraints.
For example, in some implementations, the determination module 516 checks
whether the amount of fluid directed to the accumulator 340 during charging is
about equal to the amount of fluid directed to the accumulator 340 during
discharging so that the change pressure in the accumulator 340 over the duty cycle
satisfies the following equation:
j Qaccdt = j Qacc,pdt + Qacc,p dt = 0
pEcharge pedischcrge
If the determination module 516 determines that the first threshold is
set sufficiently high so that the volume of fluid being forwarded to the accumulator
340 during charging is at least as large as the volume of fluid required to be
discharged from the accumulator 340 during the duty cycle, then the selection
process 500 performs any appropriate completion procedures and ends at a stop
module 518. If the determination modules 516 determines that an insufficient
amount of fluid is being forwarded to the accumulator 340 during charging,
however, then the first threshold level is increased at an adjust operation 520 and the
selection process 500 cycles back to the test operation 514 to begin again.
The above specification, examples and data provide a complete
description of the manufacture and use of the composition of the invention. Since
many embodiments of the invention can be made without departing from the spirit
and scope of the invention, the invention resides in the claims hereinafter appended.
CLAIMS:
1. A hydraulic drive system for driving a load, the hydraulic drive system
comprising:
a drive shaft;
first and second hydraulic pumps driven by the drive shaft, at least the
second hydraulic pump being a variable displacement bidirectional pump, the first
hydraulic pump having an outlet in fluid communication with a supply line that
connects to the load and an inlet in fluid communication with a reservoir, the second
hydraulic pump having a first pump port in fluid communication with the supply
line and a second pump port selectively in fluid communication with the reservoir
and with a hydraulic fluid accumulator;
a control system that operates the hydraulic drive system in a plurality of
modes including: a) a first mode where the second hydraulic pump pumps hydraulic
fluid from the supply line to the accumulator; b) a second mode where the second
hydraulic pump pumps hydraulic fluid from the accumulator to the supply line; c) a
third mode where the second hydraulic pump pumps hydraulic fluid from the supply
line to the reservoir; and d) a fourth mode where the second hydraulic pump pumps
hydraulic fluid from the reservoir to the supply line.
2. The hydraulic drive system of claim 1, wherein the control system operates
the hydraulic drive system in the first mode when the load is below a predetermined
lower threshold level and the accumulator is not fully charged.
3. The hydraulic drive system of claim 2, wherein the control system operates
the hydraulic drive system in the second mode when the load is above a
predetermined upper threshold level.
4. The hydraulic drive system of claim 1, wherein the control system operates
the hydraulic drive system in the third mode when the load is below a predetermined
lower threshold level and the accumulator is fully charged.
5. The hydraulic drive system of claim 3, wherein the control system operates
the hydraulic drive system in the fourth mode when the load is between the
predetermined upper and lower threshold levels.
6. The hydraulic drive system of claim 1, wherein the drive shaft is rotated by
an electric motor.
7. The hydraulic drive system of claim 1, wherein the first hydraulic pump is a
fixed displacement hydraulic pump.
8. The hydraulic drive system of claim 1, wherein the control system includes a
directional valve having a first valve port in fluid communication with the second
pump port, a second valve port in fluid communication with the accumulator and a
third valve port in fluid communication with the reservoir.
9. The hydraulic drive system of claim 8, wherein the valve is a three position
valve having a first position where the first valve port is connected to the third valve
port and the accumulator is isolated form the first valve port, a second position
where the second and third valve ports are isolated from the first valve port, and a
third position where the first valve port is connected to the second valve port and the
third valve port is isolated from the first valve port.
10. A method for operating a hydraulic drive system to drive a load, the
hydraulic drive system including first and second hydraulic pumps driven by a
common shaft, at least the second pump being a bidirectional pump, the method
comprising:
pumping hydraulic fluid from a reservoir to a supply line with both hydraulic
pumps when the load is between an upper threshold level and a lower threshold
level;
pumping hydraulic fluid from the supply line to an accumulator with the
second hydraulic pump when the load is below the lower threshold level; and
pumping hydraulic fluid from the accumulator to the supply line with the
second hydraulic pump when the load is above the upper threshold level.
11. The method of claim 10, further comprising pumping hydraulic fluid from
the supply line to the reservoir with the second hydraulic pump when the load is
below the lower threshold level and the accumulator is fully charged.
12. The method of claim 10, wherein the first hydraulic pump is a fixed
displacement pump and the second hydraulic pump is a variable displacement pump.
13. A hydraulic drive system for powering a load, the hydraulic drive system
comprising:
a pump arrangement for pumping hydraulic fluid to a supply line, the pump
arrangement being selected from a group consisting of: a) a variable displacement
pump driven by an electric motor; and b) a fixed displacement pump driven by a
variable frequency drive;
a hydraulic fluid accumulator; and
a valve for selectively connecting the hydraulic fluid accumulator in fluid
communication with the pump arrangement and for isolating the hydraulic fluid
accumulator from the pump arrangement.
14. The hydraulic drive system of claim 13, wherein the pump arrangement
includes the variable displacement pump driven by the electric motor, and wherein
first and second proportional flow valves are positioned in series along the supply
line downstream from the variable displacement pump.
15. The hydraulic drive system of claim 14, wherein the accumulator connects to
the supply line at a location between the first and second proportional flow valves.
16. Then hydraulic drive system of claim 15, further comprising a relief valve
that connects to the supply line at a location between the accumulator and the first
proportional flow valve.
17. The hydraulic drive system of claim 13, wherein the pump arrangement
includes the fixed displacement pump driven by the variable frequency drive,
wherein a proportional flow valve is positioned on the supply line downstream from
the fixed displacement pump, and wherein the accumulator connects to the supply
line between the proportional flow valve and the fixed displacement pump.
18. The hydraulic drive system of claim 17, further comprising a relief valve that
connects to the supply line at a location between the accumulator and the fixed
displacement pump.
19. The hydraulic drive system of claim 13, wherein the pump arrangement
includes a first pump and a second pump, wherein the second pump is the variable
displacement pump driven by the electric motor, wherein the variable displacement
pump is a bi-directional pump, wherein the first and second pumps are both driven
by a common drive shaft, wherein the first and second pumps both have first ports in
fluid communication with the supply line, and wherein the second pump has a
second port selectively in fluid communication with the hydraulic fluid accumulator.
20. The hydraulic drive system of claim 19, wherein the first pump has a second
port that is always in fluid communication with a reservoir, and wherein the second
port of the second pump is selectively in fluid communication with the reservoir.