applied

sciences
Article
A Positive Flow Control System for Electric
Excavators Based on Variable Speed Control
Shengjie Fu, Zhongshen Li , Tianliang Lin *, Qihuai Chen and Haoling Ren
College of Mechanical Engineering and Automation, Huaqiao University, Xiamen 361021, China;
fsj@hqu.edu.cn (S.F.); lzscyw@hqu.edu.cn (Z.L.); 11025049@zju.edu.cn (Q.C.); rhl@hqu.edu.cn (H.R.)
* Correspondence: ltl@hqu.edu.cn




Received: 11 June 2020; Accepted: 13 July 2020; Published: 14 July 2020


Abstract: Energy conservation and emission reduction of construction machinery are the focus of
current research. The traditional excavator, whose hydraulic pump is driven by the engine, has high
fuel consumption and emissions. Furthermore, it is difficult to match the working point of the engine
to that of the hydraulic pump. Current pure electric drive technology has the advantages of zero
pollution and low noise, and the motor used has the advantages of fast response and a wide speed
range. Based on the characteristics of the pure electric drive technology, a positive flow system
based on variable speed constant displacement instead of a variable displacement pump for pure
electric construction machinery is put forward to realize the flow-matching of the whole machine.
The basic structure and working principle were introduced. The control process was analyzed.
The controllability and energy saving of the proposed system were tested through simulation and
experimental analysis. The research results showed that the controllability of the proposed positive
flow system was comparable to that of the traditional throttling speed-regulating control system.
The energy-saving efficiency of the proposed positive flow system is increased by 35.2% compared to
that of the tradition control system. To further exploit the strong overload capacity of electric motors
of electric construction machinery and solve the insufficient power under sudden load, research on
constant power control will be carried out in the future.

Keywords: construction machinery; energy saving; positive flow system (PFS); throttling
speed-regulating control system (TSCS); variable speed control

1. Introduction
Construction machinery consumes much energy and has great significance to energy conservation
and emissions reduction. Various energy-saving forms have been proposed, such as hybrid power
technology, which can improve fuel efficiency to some extent, but still depend on the engine [1].
There are some problems with the engine, such as low energy conversion rate, large noise, high vibration,
and bad pollution discharge. With the development of power electronics technology, frequency control
technology, and battery-based energy storage technology, pure electric drive technology is now widely
used. The application of pure electric drive technology in construction machinery can reduce the
emissions and noise, which are the shortcomings of traditional diesel and hybrid motors. Pure
electric drive technology has the following advantages: (1) Electric energy, which is clean and efficient,
is used. It can truly achieve zero emissions and operate pollution-free, and can meet the goal of energy
conservation, emissions reduction, and sustainable development [2]. (2) The motor is used as the
power source, which can improve the energy conversion rate of the power source. (3) The motor has
good speed regulation, which can help to realize the power matching of the system, reducing energy
loss and improving the controllability and energy-saving [3,4]. (4) The motor has good short-term
overload capacity and can be applied to large burst load conditions.

Appl. Sci. 2020, 10, 4826; doi:10.3390/app10144826 www.mdpi.com/journal/applsci

Appl. Sci. 2020, 10, 4826 2 of 13

To improve the energy utilization of the construction machinery, many research institutions
and scholars have devoted themselves to studying the energy conservation of hydraulic systems.
Some technologies, such as the negative flow control system [5,6], positive flow system (PFS) [7,8],
load sensitive control system [9,10], and load port independent control system [11,12] have been
successively proposed and developed. Their purpose is to reduce overflow loss and throttling loss of
the system through flow matching to improve the energy utilization. PFS has no bypass oil return
and no oil return loss. Furthermore, there is no pressure compensator, which can add throttling loss.
Compared with other hydraulic energy saving technologies, PFS can reach the best energy saving effect.
However, in a PFS, when the actuator needs to work, it takes the pump a long time to set up pressure
to overcome the load [13]. Lin et al. studied the automatic idle control characteristics of a hydraulic
excavator based on a PFS driven by pure electricity, but they did not discuss the characteristics of the
PFS [14]. Bender et al. introduced a predictive operator modeling of a virtual prototype of a hydraulic
excavator using a PFS and analyzed the influence of different driver factors on the working cycle time
and capacity consumption, but they did not investigate the performance of the PFS [15]. Han put
forward an energy-saving technology of a fully controlled positive flow excavator, and analyzed
the energy saving property of the system. A variable pump was used in the proposed system [16].
At present, the research on the PFS of construction machinery is mainly focused on the variable pump
driven by the engine, and there is less research on quantitative pumps driven by the variable speed
control of the motor, which has excellent speed regulation performance. Using motor frequency control
to substitute the engine is dependent on the performance of both the motor and quantitative pump.
To achieve as high energy savings as possible, optimization of the hydraulic system and parameter
matching between the pump and the motor should be carried out.
The frequency control technology is adapted to the hydraulic excavator by adjusting the motor
speed to achieve flow matching between the hydraulic pump and the load. A PFS based on variable
speed control is proposed through optimizing the parameters of the hydraulic system and the motor.
Simulation and experimental analysis are used to verify the feasibility and efficiency of the proposed
PFS based on variable speed. The remainder of this paper is organized as followings: Section 2
briefly introduces the structure scheme and working principle of the proposed PFS based on motor
variable speed control. The control rules and control strategies of the PFS are introduced in Section 3.
The controllability and energy-saving performance of the proposed system are analyzed in Sections 4
and 5 by simulation and experiment. Conclusions are given in Section 6.

2. Structure and Principle

To further improve the energy saving and controllability of hydraulic excavators, a PFS based on
variable speed control for pure electric excavators is proposed, as shown in Figure 1. The power of
the motor is supplied by the power network, which minimally increases the installation cost, but can
reduce the use cost to the user. In the proposed PFS, the output flow of the hydraulic pump is adjusted
by changing the motor speed to meet the requirement of the load. Therefore, the energy saving of the
hydraulic system can be realized by reducing the flow loss.
The working process of the proposed system is as follows: when the electronic control handle
receives a signal and leaves the middle position, it outputs the control signal to the motor frequency
converter and to the pilot pressure-reducing valve through the controller. The multiway valve works
at a certain opening according to the control signal and outputs a certain pressure and flow rate to
the actuator. The sensor detects the pressure signal of the actuator and sends it to the controller.
The controller outputs the control signal to the frequency converter to control the motor speed to make
the output flow of the hydraulic pump match the requirement of the load. In this process, all the
output flow of the hydraulic pump enters the actuator. Accordingly, the relief valve, in parallel with
the main pump, which is quantitative, is used as a safety valve and there is no overflow loss. When the
handle is returned to the middle position, the hydraulic pump outputs flow directly back to the tank
through the multiway valve. The proposed system has the following advantages: (1) There is no
Appl. Sci. 2020, 10, 4826 3 of 13

overflow loss and throttling loss; therefore, the efficiency is high. (2) Due to the fast response of the
Appl. Sci. 2020, 10, x FOR PEER REVIEW 3 of 13
motor, the pressure can be quickly established to overcome the load when the handle leaves the middle
position.
middle (3) Since
position. (3)the motor
Since the has high
motor efficiency
has in a wide
high efficiency in aspeed
wide range, it can make
speed range, it canthe main
make thepump
main
have a wide range of output and adapt to different working conditions.
pump have a wide range of output and adapt to different working conditions.

Figure
Figure 1.
1. Principle
Principle diagram
diagram of
of the
the positive
positive flow
flow system
system (PFS)
(PFS) of
of an
an electric
electric excavator
excavator based
based on
on variable
variable
speed control.
speed control.

3. Control
Control Strategy
Strategy
Figure 22 is
is the
thecontrol
controlstructure
structurediagram
diagram of of
thethe
PFSPFSbased on the
based variable
on the speed
variable control
speed for a for
control purea
electric
pure excavator.
electric It includes
excavator. the following
It includes three parts:
the following three(1)parts:
Control(1) unit, which
Control consists
unit, whichofconsists
an electronic
of an
control handle
electronic and ahandle
control controller.
and (2) Volumetric speed
a controller. regulation,speed
(2) Volumetric whichregulation,
consists of awhich
frequency converter,
consists of a
a motor, and a main pump. (3) Throttling speed-regulating, which includes a
frequency converter, a motor, and a main pump. (3) Throttling speed-regulating, which includes a pilot pressure-reducing
valvepressure-reducing
pilot and a multiway valve. valveTheandcontroller
a multiway receives
valve.the
Thesignals of the
controller handlethe
receives andsignals
the sensors
of theto carry
handle
out the
and thelogic processing
sensors to carryand
outflow forecast.
the logic According
processing andtoflow
the forecast.
logic operation, the controller
According to the logicsends signals
operation,
to the
the pilot valve
controller sends andsignals
inverter. Accordingly,
to the pilot valvethe
andmotor speed
inverter. is controlledthe
Accordingly, to motor
drive pump
speed output at the
is controlled
required
to flow, output
drive pump and theatpilot valve outputs
the required pressurized
flow, and the pilotoil to the
valve multiway
outputs valve atoil
pressurized a certain opening.
to the multiway
Therefore, the required flow enters the actuator and flow-matching is realized.
valve at a certain opening. Therefore, the required flow enters the actuator and flow-matching is
According to the control structure diagram shown in Figure 2, the flow diagram of the control
realized.
strategy is designed as shown in Figure 3. The controller controls the output pressure of the pilot
pressure-reducing valve according to the control signal of the electronic control handle. The spool
displacement of the multiway valve is controlled by the output pressure of the pilot valve. Then,
the flow entering the actuator is estimated. The estimated total flow required in the actuators is:

Qt = Q1 + Q2 + · · · + Qn (1)

where Qt is the total flow required in the actuator in L/min. Qn is the required flow rate of nth actuator
working at the same time, n ≥ 1.

Figure 2. Control structure diagram of a PFS based on variable speed.

pilot pressure-reducing valve and a multiway Qt = Q1valve.
+ Q2 +The + Qn
 controller receives the signals of the handle
(1)
and the sensors to carry out the logic processing and flow forecast. According to the logic operation,
where
the Qt is thesends
controller total signals
flow required in the
to the pilot actuator
valve in L/min.Accordingly,
and inverter. Qn is the required flow rate
the motor speedof is
nth actuator
controlled
working at the same time, n ≥ 1.
to drive pump output at the required flow, and the pilot valve outputs pressurized oil to the multiway
According
valveSci.
at2020,
a certain to the estimated flow rate, the motor speed can be calculated as:
Appl. 10, 4826opening. Therefore, the required flow enters the actuator and flow-matching is
4 of 13
realized. Qt
nt = (2)
Vpη pCs
where nt is the target motor speed in rpm. Vp is the displacement of the quantitative pump (main
pump) in mL/r. ηp is the volumetric efficiency of quantitative pump (main pump). Cs is the coefficient
of the estimated speed. It is used to ensure the pump output flow has a certain margin to compensate
for oil leakage.
The output power of the pump is:

nrVp pp
Pp = (3)
60000
where Pp is the pump output power in kW. nr is the real speed of the motor taken from the frequency
Figure 2. Control structure diagram of a PFS based on variable speed.
converter in rpm. pFigure Control
p is the2.pump structureindiagram
pressure MPa. of a PFS based on variable speed.

Figure
Figure 3.
3. Flow
Flow chart
chart of
of positive
positive flow
flow control
control strategy based on
strategy based on variable
variable speed
speed control.
control.

The pump output

According power is compared
to the estimated with
flow rate, the the constant
motor power
speed can point. If the
be calculated as: pump output power
is less than the constant power point, a positive flow matching mode is adopted. Otherwise, the
system switches to the constant power mode. Qt
nt = (2)
Vp ηp Cs

where nt is the target motor speed in rpm. V p is the displacement of the quantitative pump (main
pump) in mL/r. ηp is the volumetric efficiency of quantitative pump (main pump). Cs is the coefficient
of the estimated speed. It is used to ensure the pump output flow has a certain margin to compensate
for oil leakage.
The output power of the pump is:
nr Vp pp
Pp = (3)
60000
where Pp is the pump output power in kW. nr is the real speed of the motor taken from the frequency
converter in rpm. pp is the pump pressure in MPa.
Appl. Sci. 2020, 10, 4826 5 of 13

The pump output power is compared with the constant power point. If the pump output power is
less than the constant power point, a positive flow matching mode is adopted. Otherwise, the system
Appl. Sci.to
switches 2020,
the10,constant
x FOR PEER REVIEW
power mode. 5 of 13

4. 4. SimulationResults
Simulation Results

ToTo verify
verify thethe performance
performance of proposed
of the the proposed PFS based
PFS based on variable
on variable speed speed for electric
for a pure a pure excavator,
electric
theexcavator, themodel
simulation simulation model is established
is established using AMESim,
using AMESim, shown inshown
Figurein4.
Figure 4. The parameters
The parameters of theofkey
the key components are set in accordance with the actual actuator of a 1.5 t hydraulic excavator test
components are set in accordance with the actual actuator of a 1.5 t hydraulic excavator test rig.
rig.

Figure4.4.Simulation
Figure Simulation model
model of
of PFS
PFS based
basedon
onvariable
variablespeed
speedcontrol.
control.

The
The armarmcylinder
cylinderisistaken
takenasasthe
theresearch
research object.
object. A
A typical
typicalcycle
cycleofofthe
thecylinder
cylinderextension and
extension and
recovery is studied to verify the controllability and energy-saving of the proposed PFS based
recovery is studied to verify the controllability and energy-saving of the proposed PFS based on variable on
variable
speed speed
control forcontrol for an
an electric electric excavator.
excavator. Table the
Table 1 shows 1 shows the key parameters
key parameters used
used in the in the
simulation.
simulation. Combined with the actual parameters, the safety pressure of the relief valve in the main
Combined with the actual parameters, the safety pressure of the relief valve in the main oil circuit is
oil circuit is set to 20 MPa and that of the pilot oil is set to 4 MPa.
set to 20 MPa and that of the pilot oil is set to 4 MPa.
 Appl. Sci. 2020, 10, x FOR PEER REVIEW 6 of 13
Appl. Sci. 2020, 10, x FOR PEER REVIEW 6 of 13

Table
Appl. Sci. 2020, 1. Simulation parameters of positive flow system (PFS) based on variable speed control.
10, 4826 6 of 13
Table 1. Simulation parameters of positive flow system (PFS) based on variable speed control.
Simulation Parameters Value
Simulation Parameters Value
Bore
Table 1. Simulation parameters of positive flow system diameter 63
(PFS) based on variable63 mm
speed
Bore diameter mmcontrol.
Arm cylinder Rod diameter 35 mm
Arm cylinder
Simulation Rod diameter
Parameters Value 34035 mm
Stroke mm
Stroke 340 mm
Safety pressure of main oil Bore diameter 63 mm
Safety pressure of mainArm oil
cylinder Rod diameter 35 mm2020 MPa
circuit MPa
circuit Stroke 340 mm
Safety pressure of pilot oil
Safety
Safetypressure ofmain
pressure of pilotoil
oil 4 MPa
circuitcircuit 20 MPa 4 MPa
circuit
Safety pressure of pilot oil circuit 4 MPa
Pump displacement 25 mL/r
Pump displacement
Pump displacement 25 mL/r25 mL/r
Motor rated
Motor speed
rated speed 1800 1800 rpm
rpm
Motor rated speed 1800 rpm
4.1. Controllability Analysis of Proposed PFS Based on Variable Speed
4.1.4.1.
Controllability
ControllabilityAnalysis
AnalysisofofProposed
ProposedPFS
PFS Based
Based on Variable
Variable Speed
Speed
Figure 5 shows the velocity and displacement curves of the arm cylinder during one cycle. It can
Figure
Figure 5 showsthe
5 shows thevelocity
velocityand
anddisplacement
displacement curves of of the
thearm
armcylinder
cylinderduring
duringone
onecycle.
cycle.It can
It can
be seen that the displacement increases to 340 mm at first and then starts to decrease, that is, the
be be
seenseen that
that thethe displacement
displacement increases
increases to 340
to 340 mmmm at first
at first andandthenthen starts
starts to decrease,
to decrease, that that is, piston
is, the the
piston rod first extends and is then retracted. In this process, the maximum velocity of the piston is
rodpiston
0.36
rod firstand
firstm/s
extends extends andretracted.
is then
when it is extended
is then retracted.
and 0.33In this
m/s
In this process,
process,
when the maximum
it is retracted.
the maximum
velocity
The piston
velocity
of the
moves
of the piston
piston
smoothly is 0.36ism/s
without
0.36
when m/s when it is extended and 0.33 m/s when it is retracted. The piston moves
it is extended and 0.33 m/s when it is retracted. The piston moves smoothly without stagnation.smoothly without
stagnation.
stagnation.

Figure 5. Curvesofof velocity and

and displacement of the arm cylinder.
Figure5.5.Curves
Figure Curves of velocity displacementof
velocity and displacement ofthe
thearm
armcylinder.
cylinder.

Figure
Figure 6 shows
6 shows thethe curves
curves of of motor
motor speed
speed andand pilot
pilot pressure.The
pressure. Themotor
motorspeed
speedincreases
increaseswith
withthe
Figure 6 shows the curves of motor speed and pilot pressure. The motor speed increases with
the increase
increase of the pilot pressure
of the pilot and is proportional to the pilot pressure, which conforms to the
the increase of thepressure and is proportional
pilot pressure to the pilot
and is proportional pressure,
to the which conforms
pilot pressure, to the operating
which conforms to the
operating characteristics
characteristics of the PFS of theon
based PFS based on
variable variable
speed speed control.
control.
operating characteristics of the PFS based on variable speed control.

Figure 6. Curves of motor speed and pilot pressure.

Figure6.6.Curves
Figure Curves of
of motor
motor speed
speedand
andpilot
pilotpressure.
pressure.

Figure 7 shows the flow rate curves of the pump and the two chambers of the arm cylinder. It shows
that the flow rate output from the hydraulic pump is consistent with the flow rate into the two chambers
of the arm cylinder. This indicates that flow matching is realized between the hydraulic pump and the
actuator. The results verify that the proposed PFS and control strategy have good controllability.
 Appl. Sci. 2020, 10, x FOR PEER REVIEW 7 of 13

Appl. Sci. 2020, 10, x FOR PEER REVIEW 7 of 13

Figure 7 shows the flow rate curves of the pump and the two chambers of the arm cylinder. It
shows that the flow rate output from the hydraulic pump is consistent with the flow rate into the two
Figure 7 shows the flow rate curves of the pump and the two chambers of the arm cylinder. It
chambers of the arm cylinder. This indicates that flow matching is realized between the hydraulic
shows that the flow rate output from the hydraulic pump is consistent with the flow rate into the two
Appl. Sci.
pump 10, 4826
2020,and the actuator. The results verify that the proposed PFS and control strategy have good7 of 13
chambers of the arm cylinder. This indicates that flow matching is realized between the hydraulic
controllability.
pump and the actuator. The results verify that the proposed PFS and control strategy have good
controllability.

Figure
Figure 7. Flow
7. Flow rate
rate curvesofofthe
curves thepump
pump outlet
outlet and
andthe
thetwo
twochambers of the
chambers armarm
of the cylinder.
cylinder.
Figure 7. Flow rate curves of the pump outlet and the two chambers of the arm cylinder.
The traditional
The traditional hydraulic
hydraulic excavatorisisaa fixed-speed
excavator fixed-speed fixed-displacement
fixed-displacement system, andand
system, the actuator
the actuator
speed is controlled
speed is controlled by
by the the throttling
throttling control.
control. Although the throttling control system has a large energy
The traditional hydraulic excavator is aAlthough
fixed-speedthe throttling control
fixed-displacement system
system, andhastheaactuator
large energy
loss, it has good controllability. Figures 8 and 9 show the comparison curves of the displacement and
loss, speed
it has isgood controllability.
controlled Figures
by the throttling 8 andAlthough
control. 9 show thethethrottling
comparison curves
control of has
system the adisplacement
large energy and
velocity of the arm cylinder between the proposed PFS based on variable speed control and the
loss,ofitthe
velocity hasarm
goodcylinder
controllability.
between Figures 8 and 9 show
the proposed PFS the
basedcomparison curves
on variable speedof the displacement
control and
and the throttling
throttling control system based on the constant speed constant displacement system. It can be seen
velocity
control system of the arm
basedthat cylinder
on the between the proposed PFS based on variable speed control and the
from the curves theconstant speedand
displacement constant
velocity displacement system.are
of the two systems It almost
can be seen frominthe
the same curves
one
throttling
that the control system
displacement and based on
velocity of the
theconstant
two speedare
systems constant
almost displacement
the same insystem.
one It canThis
cycle. be seen
indicates
cycle. This indicates that the proposed PFS can achieve the same controllability as the throttling
from the curves that the displacement and velocity of the two systems are almost the same in one
that the proposed PFS
speed-regulating can achieve
control the same
system (TSCS). controllability
In the as theTSCS
following figures, throttling
is shortspeed-regulating
for throttling speed-control
cycle. This indicates that the proposed PFS can achieve the same controllability as the throttling
system (TSCS).control
regulating In the system
followingand figures, TSCS
PFS is short for is short for
positive flowthrottling
system. speed-regulating control system
speed-regulating control system (TSCS). In the following figures, TSCS is short for throttling speed-
and PFS is short
regulating for positive
control system andflowPFS
system.
is short for positive flow system.

Figure 8. Displacement curves of the arm cylinder of the two systems.

Appl. Sci. 2020, 10, xFigure

FOR Displacement
8. 8.
Figure
PEER Displacementcurves
REVIEW of the
curves of thearm
armcylinder
cylinderof of
thethe
twotwo systems.
systems. 8 of 13

Figure
Figure 9. 9. Velocitycurves
Velocity curvesof
of the
the arm
arm cylinder
cylinderofofthe two
the systems.
two systems.

4.2. Energy Saving Analysis of Proposed PFS Based on Variable Speed

The energy consumption of a hydraulic system is mainly reflected in the power consumption,
and the hydraulic power is calculated by the flow and pressure. The following is the energy
consumption comparison between the PFS based on variable speed and the TSCS based on constant
speed.
 Figure 9. Velocity curves of the arm cylinder of the two systems.
Appl. Sci. 2020, 10, 4826 8 of 13
4.2. Energy Saving Analysis of Proposed PFS Based on Variable Speed
The energy
4.2. Energy Savingconsumption of a hydraulic
Analysis of Proposed PFS Basedsystem is mainly
on Variable Speedreflected in the power consumption,
and the hydraulic power is calculated by the flow and pressure. The following is the energy
The energy consumption of a hydraulic system is mainly reflected in the power consumption,
consumption comparison between the PFS based on variable speed and the TSCS based on constant
and the hydraulic power is calculated by the flow and pressure. The following is the energy consumption
speed.
comparison between the PFS based on variable speed and the TSCS based on constant speed.
Figure 10 shows the pressure and flow rate curves of the main pump outlet of the two systems.
Figure 10 shows the pressure and flow rate curves of the main pump outlet of the two systems.
As seen from the pressure curves, during the actuator’s movement, the changing trend of pump
As seen from the pressure curves, during the actuator’s movement, the changing trend of pump outlet
outlet pressure with the load in the two systems is basically the same. According to the flow curves,
pressure with the load in the two systems is basically the same. According to the flow curves, the pump
the pump output flow based on constant speed and constant displacement is almost constant, around
output flow based on constant speed and constant displacement is almost constant, around 37 L/min.
37 L/min. While the output flow of the PFS based on variable speed displacement is controlled by the
While the output flow of the PFS based on variable speed displacement is controlled by the opening of
opening of the electronic control handle, with the increasing of the handle movement amplitude, the
the electronic control handle, with the increasing of the handle movement amplitude, the output flow
output flow increases accordingly. Therefore, it can realize the flow matching between the pump and
increases accordingly. Therefore, it can realize the flow matching between the pump and the load and
the load and can reduce the energy loss. This proves that the proposed PFS has good energy-saving
can reduce the energy loss. This proves that the proposed PFS has good energy-saving ability.
ability.

(a) Pressure curves (b) Flow rate curves

Figure 10. Pressure

Figure10. Pressureand
andflow
flowrate
ratecurves
curvesof
ofthe
thepump
pumpoutlet
outletof
ofthe
thetwo
twosystems.
systems.

Figure
Figure 11 11 shows
shows the
theenergy
energyconsumption
consumption comparison
comparison curves
curves of
of hydraulic
hydraulic pumps
pumps of of the
the two
two
systems
systems in one working cycle. The energy consumption of PFS is lower than that of TSCS. In
in one working cycle. The energy consumption of PFS is lower than that of TSCS. In one
one
working cycle, in which the arm cylinder extends and retracts, the energy consumed by
working cycle, in which the arm cylinder extends and retracts, the energy consumed by TSCS is about TSCS is about
67.5
67.5kJ
Appl. kJand
Sci. andthat
2020, that of
ofPFS
10, x FORPFSisisabout
PEER about47.5
REVIEW47.5kJ.
kJ.The
Theenergy-saving
energy-savingefficiency
efficiencyisisup
uptoto29.6%.
29.6%. 9 of 13

Figure 11. Energy consumption curves of the pump in the two systems.

5. Experimental Results
5. Experimental Results and
and Discussion
Discussion
To
To further
further verify
verify the
the controllability
controllability and
and energy
energy saving
saving of
of the
the proposed
proposed PFS
PFS based
based on
on variable
variable
speed
speed control, a 1.5 t excavator test rig was built. The layout and key components of the test
control, a 1.5 t excavator test rig was built. The layout and key components of the test rig
rig are
are
shown in Figures 12 and 13. The test rig mainly includes the following parts: (1) The variable speed
power system, including the motor, main pump, and pilot pump. The dual pump is driven by the
motor. (2) The throttling speed-regulating system includes a pilot proportional pressure-reducing
valve and multiway valve. According to the control signal, it controls the flow into the actuator to
adjust the operating speed of the actuator. (3) The measurement and control system includes an
 Figure 11. Energy consumption curves of the pump in the two systems.

5. Experimental Results and Discussion

To further verify the controllability and energy saving of the proposed PFS based on variable
speed control,
Appl. Sci. 2020, 10,a4826
1.5 t excavator test rig was built. The layout and key components of the test rig9 are of 13
shown in Figures 12 and 13. The test rig mainly includes the following parts: (1) The variable speed
power system, including the motor, main pump, and pilot pump. The dual pump is driven by the
shown(2)
motor. in Figures 12 and speed-regulating
The throttling 13. The test rig mainly
system includes
includes thea following parts: (1) pressure-reducing
pilot proportional The variable speed
valve and multiway valve. According to the control signal, it controls the flow into is
power system, including the motor, main pump, and pilot pump. The dual pump thedriven by the
actuator to
motor.the
adjust (2) operating
The throttling speed-regulating
speed of the actuator.system includes
(3) The a pilot proportional
measurement and controlpressure-reducing
system includesvalvean
and multiway
industrial valve. According
PC, acquisition board,tofrequency
the controlconverter,
signal, it controls the flowhandle,
electric control into thevarious
actuatorsensors,
to adjustetc.
the
operating speed of the actuator. (3) The measurement and control system includes
The key parameters, such as displacement and pressures, are collected and processed. Moreover, the an industrial PC,
acquisition board,
proportional relief frequency converter,toelectric
valve is controlled simulatecontrol handle,
the load withvarious sensors, etc.and
the measurement Thecontrol
key parameters,
system.
suchTheas displacement and pressures, are collected and processed. Moreover, the proportional
arm cylinder is discussed in the following test. A typical working cycle consists of the piston relief
valve is controlled
extending to simulate the load with the measurement and control system.
and retracting.

Figure 12. A 1.5 t excavator test rig.

Appl. Sci. 2020, 10, x FOR PEER REVIEW Figure 12. A 1.5 t excavator test rig. 10 of 13

Figure 13.
Figure 13. Key
Key components of the
components of the test
test rig.
rig.

5.1. Controllability Analysis

The arm cylinder is discussed in the following test. A typical working cycle consists of the piston
extending
Figureand retracting.
14 shows the pilot pressure signal curves corresponding to the two chambers of the arm
cylinder. Under the control signal of the electronic control handle, the corresponding pilot pressure-
5.1. Controllability Analysis
reducing valve works and outputs the pilot pressure, while the pilot pressure corresponding to the
otherFigure
chamber14 remains
shows the pilot
zero. pressure
Within 0–3.2signal
s, the curves corresponding tovalve
pilot pressure-reducing the two chambers of
corresponding to the
the
arm cylinder.
rodless chamberUnder
works,the
andcontrol signal
the pilot of the
pressure electronic
increases from control
zero tohandle,
3.2 MPa.the
Thecorresponding pilot
cylinder completes
pressure-reducing valve works
the extension movement. and 3.5–6.5
Within outputss,the
thepilot pressure,
pilot pressure while the pilot pressure
corresponding to thecorresponding
rod chamber
to the other
increases chamber
from zero toremains
3.2 MPa,zero. Within
at which 0–3.2
time thes,cylinder
the pilotcompletes
pressure-reducing valve
the retraction corresponding
movement.
 Figure 13. Key components of the test rig.

5.1. Controllability Analysis

Figure 14 shows the pilot pressure signal curves corresponding to the two chambers of the arm
cylinder. Under
Appl. Sci. 2020, the control signal of the electronic control handle, the corresponding pilot pressure-
10, 4826 10 of 13
reducing valve works and outputs the pilot pressure, while the pilot pressure corresponding to the
other chamber remains zero. Within 0–3.2 s, the pilot pressure-reducing valve corresponding to the
to the rodless
rodless chamber chamber
works,works,
and theand
pilotthe pilot pressure
pressure increasesincreases
from zerofrom zero
to 3.2 to The
MPa. 3.2 MPa. Thecompletes
cylinder cylinder
completes the extension movement. Within 3.5–6.5 s, the pilot pressure corresponding
the extension movement. Within 3.5–6.5 s, the pilot pressure corresponding to the rod chamber to the rod
chamber
increasesincreases
from zero from zero
to 3.2 to 3.2
MPa, at MPa,
whichattime
whichthetime the cylinder
cylinder completes
completes the retraction
the retraction movement.
movement.

Figure 14. Pilot pressure curves of the two chambers of the arm cylinder.
cylinder.

Figure 15 showsshowsthe thecurves

curvesofofthe
thepilot
pilot pressure
pressure and
and motor
motor speed.
speed. TheThe motor
motor speedspeed andpilot
and the the
pilot pressure are both controlled by the electronic control handle signals at the same
pressure are both controlled by the electronic control handle signals at the same time. When the pilottime. When the
pilot pressure
pressure increases,
increases, the the motor
motor speed
speed increases
increases accordingly,
accordingly, whichisisconforms
which conformswith with the operation
operation
characteristics
characteristicsof ofthe
thePFS
PFSbased onon
based variable
variablespeed. TheThe
speed. motor speed
motor risesrises
speed rapidly, and there
rapidly, and is no speed
there is no
drop, except the motor speed fluctuates slightly at the maximum speed. This indicates
speed drop, except the motor speed fluctuates slightly at the maximum speed. This indicates that the that the output
flow
output of flow
the hydraulic pump can
of the hydraulic pump meet
canthe
meetrequirements of the of
the requirements actuator for rapid
the actuator movement.
for rapid It also
movement. It
verifies
also that
verifies the
thatproposed
the PFS
proposed
Appl. Sci. 2020, 10, x FOR PEER REVIEW based
PFS on
basedvariable
on speed
variable has
speed good
has controllability.
good controllability. 11 of 13

Figure 15. Pilot

Pilot pressure
pressure and motor speed curves of the PFS based on variable speed control.

5.2. Energy
5.2. Energy Saving
Saving Analysis
Analysis
Figure 16
Figure 16 shows
shows the
the pump
pump power
power comparison
comparison of the PFS
of the PFS based
based on
on variable
variable speed
speed andand the
the TSCS
TSCS
based on fix speed fixed displacement. It is obvious that the PFS consumes less energy than
based on fix speed fixed displacement. It is obvious that the PFS consumes less energy than the TSCSthe TSCS
during the
during the working
working process. The maximum
process. The maximum instantaneous
instantaneous power
power reduction
reduction is
is up
up to
to 33 kW.
kW.
Figure 17 shows the comparison of the pump energy consumption of the two systems.
The consumed energy of the FPS is lower than that of the TSCS. During one working cycle of
arm cylinder extension and retraction, the consumed energy by TSCS is about 73 kJ, while that of PFS
is 54 kJ. The energy-saving efficiency of the proposed PFS can reach 35.2%. The reported energy-saving
efficiency of the PFS is about 20% [17]. The experimental results prove that the proposed PFS based on
variable speed has good energy-saving ability.
 Figure 15. Pilot pressure and motor speed curves of the PFS based on variable speed control.

5.2. Energy Saving Analysis

Figure 16 shows the pump power comparison of the PFS based on variable speed and the TSCS
based on2020,
Appl. Sci. fix 10,
speed
4826 fixed displacement. It is obvious that the PFS consumes less energy than the11
TSCS
of 13
during the working process. The maximum instantaneous power reduction is up to 3 kW.

Figure 16. Pump output power curves of the two systems.

Figure 17 shows the comparison of the pump energy consumption of the two systems. The
consumed energy of the FPS is lower than that of the TSCS. During one working cycle of arm cylinder
extension and retraction, the consumed energy by TSCS is about 73 kJ, while that of PFS is 54 kJ. The
energy-saving efficiency of the proposed PFS can reach 35.2%. The reported energy-saving efficiency
of the PFS is about 20% [17]. The experimental results prove that the proposed PFS based on variable
Figure 16.
speed has good energy-saving Pump
Pump output
ability. output power
power curves of the two systems.

Figure 17 shows the comparison of the pump energy consumption of the two systems. The
consumed energy of the FPS is lower than that of the TSCS. During one working cycle of arm cylinder
extension and retraction, the consumed energy by TSCS is about 73 kJ, while that of PFS is 54 kJ. The
energy-saving efficiency of the proposed PFS can reach 35.2%. The reported energy-saving efficiency
of the PFS is about 20% [17]. The experimental results prove that the proposed PFS based on variable
speed has good energy-saving ability.

Figure 17. Consumed

Consumed energy
energy curves of the two systems.

6. Conclusions
Pure electric construction machinery uses a motor to drive a hydraulic pump. Considering the
good speed regulation performance and quick response of the motor, a PFS based on variable speed
constant displacement for pure electric construction machinery is put forward to improve energy
utilization and is verified by simulation and experiment. The results show that the proposed PFS based
Figure 17. Consumed energy curves of the two systems.
on variable speed can achieve the same controllability as a TSCS. The proposed PFS can use up to
35.2% less energy and has good energy savings compared to that of the traditional system. In addition,
the cost of the hydraulic system is reduced and the volumetric efficiency of the hydraulic pump is
improved because a quantitative pump is adopted. The use cost is also reduced because of the cheaper
price of electric energy than diesel. The overall energy utilization rate is improved to some extent.
The proposed system is not only suitable for hydraulic excavators, but also for other construction
machinery such as loaders and forklifts. To fully take advantage of the strong overload capacity of
electric motors and solve the issue of insufficient power under sudden load, constant power control
research based on PFS with variable speed will be carried out in the future.

Author Contributions: S.F. proposed the idea of positive flow system based on variable speed control.
Z.L. developed the structure and working mode. T.L. wrote the paper. Q.C. checked and edited the paper.
H.R. analyzed the data. All authors have read and agreed to the published version of the manuscript.
Funding: This research was funded by National Natural Science Foundation of China (No. 51875218 & 51905180),
Excellent Outstanding Youth Foundation of Fujian Province (No. 2018J06014), Industry Cooperation of Major
Science and Technology Project of Fujian Province (No. 2019H6015), Natural Science Foundation of Fujian Province
(No. 2018J01068 & 2019J01060), and STS project of Fujian Province (No. 2018T3015). This work also has been
supported by Fujian Southchina Heavy Machinery Manufacture Co., Ltd.
Conflicts of Interest: The authors declare no conflict of interest.
Appl. Sci. 2020, 10, 4826 12 of 13

Nomenclature
Cs coefficient of the estimated speed
nc target motor speed
nr real speed of the motor taken from the frequency converter
pp pump pressure
Pp pump output power
Qn required flow rate of nth actuator working at the same time
Qt total flow required in the actuator
Vp displacement of the quantitative pump (main pump)
ηp volumetric efficiency of quantitative pump (main pump)

Abbreviations
PFS positive flow system
TSCS throttling speed-regulating control system

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