J. Cent. South Univ. (2014) 21: 3119−3125
DOI: 10.1007/s11771-014-2283-y
Control strategy for energy recovery system in hybrid forklift
GONG Jun(龚俊)1, 2, HE Qing-hua(何清华)1, 2, ZHANG Da-qing(张大庆)2, ZHAO Yu-ming(赵喻明)1,2,
LIU Chang-sheng(刘昌盛)1, 2, TANG Zhong-yong(唐中勇)2
1. State Key Laboratory of High Performance Complicated Manufacturing,
Central South University, Changsha 410083, China;
2. National Enterprise R&D Center, Sunward Intelligent Equipment Co., Ltd., Changsha 410100, China
© Central South University Press and Springer-Verlag Berlin Heidelberg 2014
Abstract: After analyzing the working condition of the conventional diesel forklift, an energy recovery system in hybrid forklift is
considered and its simulation model is built. Then, the control strategy for the proposed energy recovery system is discussed, which
is validated and evaluated by simulation. The simulation results show that the proposed control strategy can achieve balance of the
power and keep the state of charge (SOC) of ultra capacitor in a reasonable range, and the fuel consumption can be reduced by about
20.8% compared with the conventional diesel forklift. Finally, the feasibility of the simulation results is experimentally verified
based on the lifting energy recovery system.
Key words: hybrid power; forklift truck; energy recovery; control strategy; ultra capacitor
1 Introduction
Energy efficiency is gaining importance in all field
of engineering [1−3]. As a typical handling equipment,
forklift is widely used in construction machinery. Due to
the large number of forklifts used in the world, even a
small energy saving in one device would means a large
energy saving in total. Therefore, research on the energy
saving of the forklift is beneficial to relax global energy
crisis and environmental pollution.
Traditional fossil fuel forklifts use engine to supply
mechanical energy to rotate a hydraulic pump. The
control of hydraulics is realized with control valves.
Energy is used for all movements, but none is recovered
back. Lifting and lowering of cargo, speeding up and
down of vehicle frequently are distinct running
characteristic of forklift, which wastes a great deal of
energy. Accumulator in hybrid system supplies the
condition for energy recovery of the fossil fuel forklift.
The maturing application of hybrid technology in
automobiles [4−5] and construction machinery [6−7]
provides reference for energy recovery in hybrid forklift.
Nowadays, much attention has been put into the
evaluation and analysis of energy saving potential and
recuperation capability [8−12]. In fact, one of the most
important problems when energy recovery system is
applied to hybrid system is the distribution control
strategy of the multi-source power. Similar researches
exist in the hybrid excavator [13−14] and vehicle[15−16].
There are many approaches to the energy distribution.
Such as model-based optimal control strategies [17−18]
and ruled-based strategies [19−20]. The former can
obtain off-line power distribution map by calculating the
equivalent system efficiency. Due to the complex
calculator, it is difficult to achieve real-time control. The
main benefit of rule-based strategies is the low hardware
requirement to control system. However, rule-based
strategies require much experiment data to match the
specific configuration. Due to the differences in working
condition and system structure which are analyzed in
Part 2, the existing research results can’t be used for the
hybrid forklift directly.
In this work, we concentrate on the power control
strategy of energy recovery in hybrid forklift. The
working condition of forklift is analyzed, a driving and
energy recovery system scheme of hybrid forklift is
proposed, and the system simulation model is then
established. Secondly, a rule-based control strategy is
proposed, which is evaluated by simulating. At last, the
energy recovery system test stand is constructed, and
potential energy recovery system of the forklift truck is
analyzed and tested.
Foundation item: Project(2013BAF07B02) supported by National Science and Technology Support Program of China
Received date: 2013−07−22; Accepted date: 2013−11−12
Corresponding author: GONG Jun, PhD Candidate; Tel: +86−15116338642; E-mail: [email protected]
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2 System structure and modeling
2.1 Working condition analysis
As mentioned above, frequent lifting, lowering,
acceleration and deceleration are the important
characteristics of forklift. On the basis of analyzing test
data of existing 3-ton forklift, the power schematic
diagram of forklift under typical stacking condition is
obtained, as shown in Fig. 1. As we can see from Fig. 1,
during the working cycle of 40 s, the forklift
accomplishes cargo lowering (process A), back running
(process B), back braking (process C), forward running
(process D), forward braking (process E) and cargo
lifting (process F). Shadows B, D and F are the
demanded powers. Shadows A, C and E are the braking
powers, which are the recyclable powers in theory.
Compared with vehicle and other construction
machinery, the power of forklift fluctuates is more
severely and frequently.
Fig. 1 Power schematic diagram of forklift under typical
stacking condition
2.2 System structure and principle
The structure of the hybrid forklift is shown in
Fig. 2, the engine and the assist motor (AM) drive the
pump/motor (P/M) in a parallel hybrid style. Ultra
capacitor is used as the storage unit. An electromagnetic
clutch is equipped to control the power flow.
In this system, on the one hand, the AM assists the
engine to work as a second power. When the demanded
power of the P/M oversteps the power limit of engine,
the AM works in motor mode to drive the P/M with the
engine together; when the demanded power of the P/M is
lower than the economic output power of engine, the AM
works in generator mode to absorb the redundant power
of engine, which is beneficial to obtain a better fuel
economy. On the other hand, in the process of cargo
lowing, the clutch is released to keep the engine in idling.
The P/M works in hydraulic motor-mode to output
mechanical power and the AM works in generator-mode
Fig. 2 System structure of hybrid forklift
to transform the mechanical energy into electrical energy
and storage in ultra capacitor.
Driving system is only powered by electricity. In the
braking process, the diving motor (DM) works in
generator mode to absorb the kinetic energy of the
vehicle. When the DM demands power, corresponding
power is provided by the ultra capacitor or the AM
depending on the actual control strategy.
2.3 Simulation model
In order to evaluate hybrid forklift system and the
proposed control strategy, a simulation model is built in
MATLAB/Simulink, as shown in Fig. 3. In this model,
the steady state fuel consumption characteristics of the
engine, the efficiency of AM and DM, and the internal
resistance of the ultra capacitor are taken into account in
detail. Pressure and flow loss of hydraulic unit and
friction of the lifting mechanism are treated equivalently.
3 Power control strategy
According to the analysis above, the power
distribution of power system becomes more flexible with
a result of the introduction of hybrid system. Different
power distribution strategies influence the fuel economy
significantly. Restricting the engine within the high
efficient region, achieving the maximization of energy
recovery, and keeping the safe state of ultra capacitor are
the main purposes of power control strategy in hydraulic
forklift.
3.1 Details of control strategy
Figure 4 shows the block diagram of the power
controller of the hybrid forklift. The power controller
should provide the balance of the instantaneous power
among the engine, the hydraulic system, the driving
system and the ultra capacitor. And also, it should keep
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Fig. 3 Block diagram of system simulation model
the balance of the ultra capacitor energy during a
working period.
In Fig. 4, nengine is engine speed, Peng_max is the
power value when the engine is working at the maximum
efficiency point under the corresponding speed, PP/M is
the demanding power of the P/M. The P/M works as a
motor and drives the AM to generate when PP/M is
negative. Hcap and Lcap are the maximum and minimum
limit values of the ultra capacitor, respectively.
Similarly, HAM and LAM represent the upper and
lower limit values of the AM power demand, respectively.
Flag 1 represents the clutch state. Whether the power of
the DM is supplied by the ultra capacity or the AM
Fig. 4 Block diagram of energy management controller
depends on the value of the Flag 2. These values are
decided by the rules shown in Fig. 5.
If the P/M power is negative, the P/M works as a
motor, correspondingly, the clutch is in the releasing
state and the DM is powered by the ultra capacitor, as
rule 1 shown in Fig. 5. If the P/M power is bigger than
zero, and the SOC of ultra capacity is safe, the
demanding power of P/M will be supplied by the engine
and demanding power of the DM will be supplied by
super capacity, as rule 2 shown in Fig. 5. If SOC of ultra
capacity is larger than the maximum SOCmax and the DM
is in decreasing process, the AM will absorb the braking
power which is used to assist engine to drive the P/M, as
rule 3 shown in Fig. 5. If SOC of ultra capacity is larger
than the maximum SOCmax but the DM is in speedup
process, the ultra capacity will power the DM, as rule 5
shown in Fig. 5. If SOC of ultra capacity is less than the
minimum SOCmin and the DM works as a generator, ultra
capacity absorbs the power generated by the DM, as
rule 4 shown in Fig. 5. If SOC of ultra capacity is less
than the minimum SOCmin but the DM works as a motor,
the AM works as a generator and supplies the DM with
power. If SOC of ultra capacity is less than the minimum
SOCmin and the DM works as a motor, the AM works as
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Fig. 5 Flowchart for decision of feed forward power command of assist motor and ultra capacitor
a generator and supplies the DM with power, as rule 6
shown in Fig. 5.
From top to bottom in Fig. 7, it shows the power of the
P/M and the DM, the open-circuit voltage and working
3.2 Simulation
Simulation is done with the parameters of a 3-ton
hybrid forklift, as listed in Table 1. The working
condition is in conformity to “Counterbalanced forklift
test methods” (JBT 3300—2010). Rated lifting cargo is
3-ton and the running route of forklift standard work is
shown in Fig. 6.
Firstly, a process of lifting and lowering is handled
at position A'. Secondly, it runs back along the route 1,
and runs forward along the route 2 until arriving at
position B'. Thirdly, a process of lifting and lowering is
handled again at position B'. Finally, vehicle runs back to
A' along the routes 3 and 4. The running distance L0 is
30 m, and the backward running distance is not restricted
specially. The lifting height in positions A' and B' is
2000 mm and the height between the cargo and the
ground is kept 300 mm during running.
Figure 7 shows the simulation result of the energy
recovery system with the power controller in Fig. 5.
Table 1 Main parameters of vehicle power system
Item
Parameter
Value
Whole
machine
Total mass/kg
4350
Wheel radius/2.54 cm
15
Hydraulic
system
Main decelerate ratio
26.05
−1
Pump/ motor volume/(mL·r )
28
Lifting-cylinder radius/mm
50
Output volume/L
Rated power/kW, Rev
speed/(r·min−1)
Maximize torque/(N·m)
2.54
37, 2650
Rated power/kW, peak power/kW
15, 30
Rated rev. speed/(r·min−1)
2000
Driving
motor
Rated power/kW, peak power/kW
30, 60
Rated rev. speed/(r·min )
2600
Ultra
capacitor
Rated voltage/V
97.2
Capacity/F
165
Engine
Assist motor
−1
148
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Fig. 6 Running route of forklift standard work
voltage of the ultra capacitor, the power of the ultra
capacitor and the AM, the state of the clutch. The values
of the power are normalized by the maximum engine
power. The high limit voltage and the low limit voltage
are set to 65 V and 95 V, respectively.
In stage 1, since the power of the P/M is larger than
zero and voltage of ultra capacitor is in the safe range,
rule 2 in Fig. 5 is applied. The clutch is in connecting
state, and the power of the P/M is supplied by the engine.
In stage 2, since the power of the P/M is smaller than
zero and voltage of ultra capacitor is in the safe range,
rule 1 in Fig. 5 is applied. Thus, the clutch is in released
state, the engine idle, the AM works in generator mode
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voltage of capacitor is lower than the low limit voltage
and the power of the DM is larger than zero, rule 4 in
Fig. 5 is applied. Thus, the clutch is in connecting state,
the power of the DM is supplied by the AM which is
driven by the engine. In this way, over-discharge of ultra
capacitor is avoided. In stage 4, since voltage of ultra
capacitor is low and the power of the DM is smaller than
zero, rule 6 in Fig. 5 is applied. Thus, the clutch is in
connecting state, the power of the DM absorbs by the
ultra capacitor. In this way, voltage of ultra capacitor is
drawn to the safe range.
The comparison between fuel consumption of the
conventional diesel forklift and the hybrid one with the
proposed control strategy is shown in Fig. 8(a). In
Fig. 8(a), about 20.8% of the reduction of the fuel
consumption with this strategy is obtained. Figure 8(b)
represents the contributions of the reasons for fuel
reduction. It can be known that the energy recovery is the
main means to improve the fuel economy in hybrid
forklift.
4 Test
4.1 Test system
In order to study the performance of energy
recovery system in hybrid forklift, a test system based on
electrical forklift is built. The schematic diagram is
Fig. 7 Simulation results of energy recovery system with proposed control strategy: (a) Hydraulic pump power & driving motor
power; (b) Open circuit voltage & working voltage of cap; (c) Ultra capacitor power & assist motor power; (d) State of clutch
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3) Lower machine sends signals to motor controller
and enables the motor state and the mode of motor speed
control. Hydraulic motor works in the pump mode and
drives the cargo to lift until the pressure of lifting
cylinder is up to relief value.
4) Lower machine sends signals to motor controller
and enables the generator state and the mode of motor
torque control. Hydraulic motor transforms the
gravitational potential energy of cargo to mechanical
energy.
5) Test is operated 10 cycles.
Fig. 8 Comparison result of fuel consumption (a) and pie chart
of reason of improvement (b)
shown in Fig. 9. Energy storage unit consists of four
ultra capacitor modules which are connected with two
series and two parallels. A single module has a capacity
of 165 F, and its rated voltage is 48 V. A hydraulic gear
motor with two working modes is equipped to be the
energy recovery device. A special controller DCF-II
(Inter Control Company) is used as the lower machine,
which is responsible for data collection and process
control of energy recovery system. Through the CAN
bus, it sends sensor and state data to upper monitor,
which is responsible for displaying the system state,
creating and saving the data file.
4.3 Experiment result
1) Hydraulic energy loss.
Figure 10 shows the pressure and flow in a lifting
and lowering process of cargo with the energy recovery
system. Curves a and b represent the pressure of pump
outlet and lifting cylinder bottom, respectively. Curve c
is the flow of pump outlet. As can be seen from the graph,
in the lifting process (3−10 s), the difference A'' between
a and b represents the pressure loss in lifting process. In
the lowering process (12−19 s), the difference C'' is the
pressure loss in lowering process. The difference B''
reflects the friction loss of the lifting mechanism. The
difference D'' illustrates the existing of flow loss.
Fig. 10 Pressure and flow of energy recovery system
Fig. 9 Schematic diagram of experiment system
4.2 Experiment method
1) Voltage of ultra capacitor is adjusted to the
working range of the electrical motor by the external
charging and discharging device.
2) Weak-current circuit is powered up. Pre-charge
relay is on the first and then the main relay is on
according to the power up process of the electrical motor.
The results show that the energy recovery system’s
largest energy loss link for lifting valve pressure loss,
accounts for about 38.8% of the lifting energy, which is
good agreement with the simulation result.
2) Energy recovery efficiency
Figure 11 shows the voltage and current of ultra
capacitor in a lifting and lowering working condition.
The initial voltage is 90 V. The voltage drops to 69 V at
the end of lifting process because of the electricity output.
At the end of recovery process, the voltage is back to
78 V. Due to the influence of the internal resistance, the
capacitor voltage can’t directly reflect the fluctuation of
capacitor energy. With the condition that internal
resistance of the capacitors unit is 7 mΩ, the SOC of the
capacitors unit is shown in Fig. 12. Relative to the
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consumption energy, the energy recovery rate was 34.6%
in the test system.
[2]
[3]
[4]
[5]
[6]
Fig. 11 Voltage and current of ultra capacitors unit
[7]
[8]
[9]
[10]
[11]
Fig. 12 SOC of ultra capacitors unit
[12]
5 Conclusions
[13]
1) An energy recovery system of hybrid forklift is
proposed based on the actual working condition, and the
corresponding simulation model is built. In addition, a
rule-based power control strategy is presented, which is
proved feasible and effective in simulation. About 20.8%
of the reduction of the fuel consumption with this
strategy is expected.
2) In order to verify the simulation result, a test
system of energy recovery is built based on a 3-ton
electrical forklift. The test result shows that pressure loss
is the largest energy loss in the energy recovery system,
and energy recovery system can reach 34.6% of the
energy recycle efficiency, which is consistent with the
simulation result.
3) The implementation of the prototype 3-ton hybrid
forklift equipped with potential energy recovery system
is in progress to show the further performance of the
proposed system and control strategy.
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(Edited by DENG Lü-xiang)