Xu System Efficiency

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Comparison of energy-saving on the speedcontrol of the VVVF hydraulic elevator

with and without the pressure accumulator

Xu Bing a, Yang Jian b,*, Yang Huayong a

a State Key Laboratory of Fluid Power Transmission and Control, Zhejiang University,

Hangzhou, 310027 Zhejiang, PR Chinab College of Urban Railway Transportation, Shanghai University of Engineering Science,

350 Xianxia Road, Shanghai 200336, PR China

Accepted 31 May 2005

Abstract

The speed control system of variable voltage variable frequency (VVVF) hydraulic elevatorwith the pressure accumulator has been designed, which is a kind of the innovation energy-saving hydraulic elevator. The control system of the VVVF hydraulic elevator with the pres-sure accumulator is analyzed, and the speed control of the VVVF hydraulic elevator with thepressure accumulator is studied using the PID control algorithm. The comparison of theexperimental researches of energy-saving is carried out for the speed control of the VVVFhydraulic elevator with and without the pressure accumulator. The experimental results showthat the VVVF hydraulic elevator with the pressure accumulator has higher efficiency com-

pared with the VVVF hydraulic elevator without the pressure accumulator. 2005 Elsevier Ltd. All rights reserved.

Keywords: VVVF hydraulic elevator; Energy-saving; Pressure accumulator; Comparison

0957-4158/$ - see front matter 2005 Elsevier Ltd. All rights reserved.doi:10.1016/j.mechatronics.2005.06.009

* Corresponding author. Address: College of Urban Railway Transportation, Shanghai University ofEngineering Science, 350 Xianxia Road, Shanghai 200336, PR China. Tel.: +86 21 621901276; fax: +86 2162190127.

E-mail address: [email protected] (J. Yang).

Mechatronics 15 (2005) 11591174
mailto:[email protected]:[email protected]


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1. Introduction

Hydraulic power transmission had a record of longer history in the applications

of industry. But its energy consumption is larger, so it is difficult to be widely appliedin the industry. In order to improve the efficiency of hydraulic power transmission,researchers have put forward many methods aiming at the energy-saving. The com-

Nomenclature

Mh equivalent mass (kg)fL equivalent frictional force (N)Ah area of piston (m

2)pLWU load pressure in the upward direction without the pressure accumulator

(MPa)pLWD load pressure in the downward direction without the pressure accumu-

lator (MPa)vWU speed of cabin in the upward direction without the pressure accumula-

tor (m/s)vWD speed of cabin in the downward direction without the pressure accumu-

lator (m/s)pLAU load pressure in the upward direction with the pressure accumulator

(MPa)pLAD load pressure in the downward direction with the pressure accumulator

(MPa)vAU speed of cabin in the upward direction with the pressure accumulator

(m/s)vAD speed of cabin in the downward direction without the pressure accumu-

lator (m/s)pAmU working pressure of motor when hydraulic pumpmotor in the accu-

mulator circuit works at the state of motor (MPa)nAmU working revolution of motor when hydraulic pumpmotor in the accu-

mulator circuit works at the state of motor (rpm)QxlU flow rate of internal leakage of motor when hydraulic pumpmotor in

the accumulator circuit works at the state of motor (m3/s)pApD working pressure of pump when hydraulic pumpmotor in the accumu-

lator circuit works at the state of pump (MPa)nApD working revolution of pump when hydraulic pumpmotor in the accu-

mulator circuit works at the state of pump (rpm)

qApD rated displacement of pump when hydraulic pumpmotor in the accu-mulator circuit works at the state of pump (m3/rev.)QxlD flow rate of internal leakage of pump when hydraulic pumpmotor in

the accumulator circuit works at the state of pump (m3/s)cxs internal leakage coefficient of pumpmotor in the accumulator circuit

1160 B. Xu et al. / Mechatronics 15 (2005) 11591174


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ponent design, control system itself and control algorithm were studied for the sys-tem of hydraulic power transmission [13]. The energy-saving of hydraulic systemwas studied using the programmable valve, which is a unique combination of five

proportional cartridge valves connected in some ways [4,5]. The combination of ahydraulic pump with a variable speed electric motor increases the efficiency of mo-tion control system [6].

With the rapid development of the modern science and technology, especially theperfection of electro-hydraulic proportional technology, hydraulic elevator with elec-tro-hydraulic proportional valves appeared [7]. In recent years the designers gradu-ally adopt the methods of pressureflow ratedisplacement and electro-calibration inthe design of hydraulic elevator, which makes hydraulic elevator more perfect insteady-state precision, dynamic response and high stability. With the developmentof computer control, microcomputer processing also begins to be applied to the con-

trol of hydraulic elevator so as to realize modern control strategy. Hydraulic elevatoris based on the technique cores of VVVF. The perfect control of the VVVF hydraulicelevator is realized through combining the control of motor revolutions with flowrate transducer or the speed transducer of good performance. It is volume controlsystem with energy-saving and without relief losses [8].

In this paper, the speed control system of the VVVF hydraulic elevator with thepressure accumulator has been designed, which is a kind of innovation energy-savinghydraulic elevator. The speed control of the VVVF hydraulic elevator with pressureaccumulator is studied. The comparison of the experiments of energy-saving are car-ried out for the speed control of the VVVF hydraulic elevator with and without thepressure accumulator. The experimental results show that in the downward direc-tion, the efficiency of the VVVF hydraulic elevator with the pressure accumulatoris 60%, and the efficiency of the VVVF hydraulic elevator without the pressure accu-mulator is only 37%, and in the upward direction, the efficiency of the VVVFhydraulic elevator with the pressure accumulator is 57%, and the efficiency of theVVVF hydraulic elevator without the pressure accumulator is only 44%.

2. Control system of the VVVF hydraulic elevator with the pressure accumulator

As shown in Fig. 1, the control system of the VVVF hydraulic elevator with pres-sure accumulator consists of two parts. The first is the main circuit containinghydraulic pumpmotor 2. The second is the accumulator circuit containing hydraulicpumpmotor 3. The purpose of the accumulator circuit is that additional power issupplied for the VVVF hydraulic elevator in the upward direction, and in the down-ward direction the pressure energy of cabin itself is stored in the pressure accumula-tor for the next use.

While the elevator begins to ascend, the upward signal is sent by the computercontroller. In the accumulator circuit, hydraulic pumpmotor 3 (hydraulic motor)

provides the additional power for electrical motor 1. If the additional power ofthe accumulator is larger than that of the payload, inverter is demanded to provideelectromagnetic braking torque and avoid the startup impacting upward, otherwise,

B. Xu et al. / Mechatronics 15 (2005) 11591174 1161


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inverter is demanded to provide driving power to finish the process of startup accel-eration. In the main circuit, hydraulic pumpmotor 2 is working at the state of

hydraulic pump. When the output pressure of the pump is larger than that of cabinitself, the hydraulic oil opens a one-way valve 11 and flows into the cylinder to makethe piston 9 move upward. When the cabin is close to the stopping position, the stop-ping signal is sent by the computer controller, and onoff solenoid valve 6 is turnedoff, then the output signal of inverter stops at the same time and the elevator stops.

While the elevator begins to run down, the pressure pre-balance is controlled firstat both sides of the one-way valve 11. When the control of the pressure pre-balanceis finished, the hydraulic elevator starts to descend, and hydraulic pumpmotor 2 isworking at the state of hydraulic motor and some pressure energy of cabin itself isstored in the pressure accumulator for the next use. The control signal given by

the computer controller drives the electrical motor 1 to run from a positive directionto a reverse direction. The electrical motor 1 begins to generate electricity at thistime. During the entire downward process, the computer controller continuously cal-

Computer controller

P2

Cabin

Inverter

Signals

Ac

cumulator

511

10

14

7

6

13

4

3 2

9

12

8

M

1

Fig. 1. Control system of the VVVF hydraulic elevator with the pressure accumulator: (1) electricalmotor; (2) hydraulic pumpmotor for main circuit; (3) hydraulic pumpmotor for accumulator circuit; (4)and (14) one way valves; (5) and (13) safety valves; (6) onoff solenoid valve; (7), (10) and (12) pressuretransducers; (8) photoelectric encoder; (9) piston; (11) pilot one way valve.



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the practical speed, curve 3 is the working pressure and curve 4 is the accumulatorpressure. From curve 4, the accumulator pressure is decreased with the increase ofthe running height of the elevator from 17 MPa to 13.5 MPa. At the stage of high

speed in curve 2, the working pressure is seriously affected and the fluctuation isslightly larger as shown in curve 3, but the working pressure is not almost affectedat the stage of low speed. The fluctuation of the accumulator pressure is less thanthat of the working pressure.

The principle of the upward direction of the elevator is that the accumulator andelectrical motor supply the output torque at the same time to drive the pump in themain circuit, which drives the piston of the cylinder to ascend. The reason for thepressure fluctuation is that the overlapping of the fluctuation of electrical motor tor-que and the fluctuation of the hydraulic motor torque in the accumulator circuit re-sults in larger working pressure fluctuation, and the reason for smaller pressure

fluctuation of the accumulator completely results from the hydraulic motor itselfin the accumulator circuit.

3.2. Experiments of the downward direction

The experimental results of the downward direction of the elevator under the con-dition of 680 kg are shown in Fig. 3. Curve 1 is the desired speed, curve 2 is the prac-tical speed, curve 3 is the working pressure, and curve 4 is the accumulator pressure.From curve 4, the accumulator pressure is increased with the decrease of the runningheight of the elevator from 13.5 MPa to 17 MPa. At the stage of high speed in curve2, the accumulator pressure is seriously affected and the pressure fluctuation isslightly larger as shown in curve 3, but the accumulator pressure is not almost af-fected at the stage of low speed. The fluctuation of the working pressure is less thanthat of the accumulator pressure.

0 12 15 18-0.1

0.0

0.1

0.2

0.3

0.4

0.5

0.6

-3

0

3

6

9

12

15

18

t

[s

]

v[m/s]

p[MPa]

4

3

21

3 6 9

Fig. 3. Experimental results of downward direction: (1) desired speed; (2) practical speed; (3) workingpressure; (4) accumulator pressure.



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The principle of the downward direction of the elevator is that electrical motorand the motor in the main circuit supply the output torque at the same time to drivethe pump in the accumulator circuit, and the energy of the pump in the accumulator

circuit is stored in the accumulator for the next use. The reason for the pressure fluc-tuation is that the overlapping of the fluctuation of electrical motor torque and thefluctuation of the hydraulic motor torque in the main circuit results in larger pressurefluctuation of the accumulator, and the reason for smaller fluctuation of the workingpressure completely results from the hydraulic motor itself in the main circuit.

4. Experiments of energy consumption of the VVVF hydraulic elevator

with the pressure accumulator [9]

4.1. Experiments of the upward direction

The experiments of the upward direction of the elevator with the pressure accu-mulator are shown in Fig. 4. Curve 1 is the load power, curve 2 is the output powerof hydraulic pumpmotor in the main circuit, curve 3 is the practical speed, curve 4is the input power of electrical motor, and curve 5 is the output power of theaccumulator.

The output power of electrical motor and the additional power of the pressureaccumulator are used to drive the elevator to ascend. The one shaft connection be-tween electrical motor and hydraulic pump results in the greater losses of frictionaltorque, furthermore, there are great energy losses of components in the system andthe system itself, so the elevator needs power supply to ensure the running of the ele-vator. With the increase of the running height of the elevator, the additional power

0 3 12 15 18-3

0

3

6

9

12

15

18

-0.2

0.0

0.2

0.4

0.6

0.8

1.0

1.2

t

[s

]

v[m/s]

W[kw]

4

3

5

1

2

6 9

Fig. 4. Experimental results of upward direction (1) load power; (2) output power of hydraulic pumpmotor; (3) practical speed; (4) input power of electrical motor; (5) output power of the accumulator.



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of the accumulator is gradually decreased, but input power of electrical motor isgradually increased, especially, it is more obvious at the stage of constant speed.


The experiments of the downward direction of the elevator with the pressure accu-mulator are shown in Fig. 5. Curve 1 is the load power, curve 2 is the input power ofhydraulic pumpmotor in the main circuit, curve 3 is the practical speed, curve 4 isthe input power of electrical motor and curve 5 is the input power of the accumula-tor. With the decrease of the running height of the elevator, the input power ofhydraulic pumpmotor in the main circuit is gradually increased, and also the storedpower of the pressure accumulator and input power of electrical motor are graduallyincreased.

5. Experiments of energy consumption of the VVVF hydraulic elevator

without the pressure accumulator [9]

5.1. Experiments of the upward direction

The experiments of the upward direction of the elevator without the pressureaccumulator are shown in Fig. 6. Curve 1 is the load power, curve 2 is the outputpower of hydraulic pumpmotor in the main circuit, curve 3 is the practical speedcurve, and curve 4 is the input power of electrical motor.

The obvious characteristic of the VVVF hydraulic elevator without the pressureaccumulator is adaptability of input energy, that is, load needs the magnitude of en-ergy which electrical motor supplies. So the VVVF hydraulic elevator without the

0 3 12 15 18-3

0

3

6

9

12

15

18

-0.2

0.0

0.2

0.4

0.6

0.8

1.0

1.2

t[s]

v[m/s]

W[kw]

5

1 2

3 4

6 9

Fig. 5. Experimental results of downward direction: (1) load power; (2) input power of hydraulic pumpmotor; (3) practical speed; (4) input power of electrical motor; (5) input power of the accumulator.



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pressure accumulator is actually a sensitive system of pressure and flow rate and isalso called sensitive system of power.


The experiments of the downward direction of the elevator without the pressureaccumulator are shown in Fig. 7. Curve 1 is the load power, curve 2 is the inputpower of hydraulic pumpmotor in the main circuit, curve 3 is the practicalspeed, and curve 4 is the feedback power of inverter. The difference between the up-ward direction and the downward direction of the elevator without the pressure

0 12 15-4

0

4

8

12

16

20

24

-0.1

0.0

0.1

0.2

0.3

0.4

0.5

0.6

t[s]

v[m/s]

W[kw]

3

4

1

2

3 6 9

Fig. 6. Experimental results of upward direction: (1) load power; (2) output power of hydraulic pumpmotor; (3) practical speed; (4) input power of electrical motor.

0 12 15-2

0

2

4

6

8

10

12

-0.1

0. 0

0. 1

0. 2

0. 3

0. 4

0. 5

0. 6

t

[s

]

v[m/s]

W[kw]

1

4

23

3 6 9

Fig. 7. Experimental results of downward direction: (1) load power; (2) input power of hydraulic pumpmotor; (3) practical speed; (4) feedback power of inverter.



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accumulator is that the feedback power in the downward direction replaces inputpower of electrical motor in the upward direction.

6. Comparison

6.1. Calculations of the power of the VVVF hydraulic elevator

without the pressure accumulator [8]

6.1.1. Load power

Load power in the upward direction is given as

WLWU Mh g vWU pLWU Ah fL vWU 1

Load power in the downward direction is given as

WLWD Mh g vWD pLWD Ah fL vWD 2

The parameters pLWU, pLWD, vWU and vWD can be measured with the pressure trans-ducers and the photoelectric encoder, then WLWU and WLWD are calculated usingformulae (1) and (2) by the computer.

6.1.2. Input power of electrical motor in the upward direction

The input power WmWU can be measured by clip-on wattmeter.

6.1.3. Feedback power in the downward directionThe feedback power WfWD can be also measured by clip-on wattmeter.

6.2. Calculations of the power of the VVVF hydraulic elevator

with the pressure accumulator

6.2.1. Load power

Load power in the upward direction is given as

WLAU Mh g vAU pLAU Ah fL vAU 3

Load power in the downward direction is given as

WLAD Mh g vAD pLAD Ah fL vAD 4

The parameters pLAU, pLAD, vAU and vAD can be measured with the pressure trans-ducers and the photoelectric encoder, then WLAU and WLAD are calculated usingformulae (3) and (4) by the computer.

6.2.2. Input power of electrical motor in the upward direction

The input power WmAU can be measured by clip-on wattmeter.

6.2.3. Input power of electrical motor in the downward direction

The input power WmAD can be also measured by clip-on wattmeter.



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6.2.4. The input power of motor is given in formula (5) in the upward direction

when hydraulic pumpmotor in the accumulator circuit works

at the state of motor

WAmU pAmU nAmU qApD QxlU

pAmU nAmU qApD cxsqApD

l pAmU

5

The parameters pAmU and nAmU can be measured, then WAmU is calculated usingformula (5) by the computer.

6.2.5. The output power of pump is given in formula (6) in the downward direction

when hydraulic pumpmotor in the accumulator circuit works at the state of pump

WApD pApD nApD qApD QxlD

pApD nApD qApD cxsqApD

l pApD

6

The parameters pApD and nApD can be measured, then WApD is calculated usingformula (6) by the computer.

6.3. Definitions of energy consumption without the pressure accumulator

6.3.1. Energy consumption of loadThe energy consumption of load in the upward direction means the energy ofdriving the elevator in formula (7).

PLWU

Zt0

WLWU dt 7

The energy consumption of load in the downward direction means that the eleva-tor itself possesses potential energy in formula (8).

PLWD

Zt0

WLWD dt 8

6.3.2. Input energy of electrical motor in the upward direction

The input energy of electrical motor means energy of driving electrical motor inthe upward direction in formula (9).

PmWU

Zt0

WmWU dt 9

6.3.3. Feedback energy in the downward direction in formula (10)

PfWD

Zt0

WfWD dt 10



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6.4. Definitions of energy consumption with the pressure accumulator

6.4.1. Energy consumption of load

The energy consumption of load in the upward direction means the energy ofdriving the elevator in formula (11).

PLAU

Zt0

WLAU dt 11

The energy consumption of load in the downward direction means that the eleva-tor itself possesses potential energy in formula (12).

PLAD

Zt0

WLAD dt 12

6.4.2. Input energy of electrical motor

The input energy of electrical motor in the upward direction in formula (13)

PmAU

Zt0

WmAU dt 13

The input energy of electrical motor in the downward direction in formula (14)

PmAD

Zt0

WmAD dt 14

6.4.3. Energy of hydraulic pumpmotor in the accumulator circuit

Under the condition of the upward direction, the energy means the output addi-tional energy of motor in formula (15) when hydraulic pumpmotor in the accumu-lator circuit works at the state of motor.

PAmU

Zt0

WAmU dt 15

Under the condition of the downward direction, the energy means the output

additional energy of pump in formula (16) when hydraulic pumpmotor in the accu-mulator circuit works at the state of pump.

PApD

Zt0

WApD dt 16

6.5. Calculations of energy consumption

6.5.1. Calculations of the energy consumption of the VVVF hydraulic elevator

without the pressure accumulator

Under the condition of the upward direction, the energy consumption of the VVVFhydraulic elevator without the pressure accumulator is calculated using formulae (7)and (9), and calculation results of the energy consumption is shown in Table 1.



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Under the condition of the upward direction, the energy consumption of theVVVF hydraulic elevator without the pressure accumulator is calculated using for-mulae (8) and (10), and calculation results of the energy consumption is shown inTable 1.

6.5.2. Calculations of the energy consumption of the VVVF hydraulic elevator

with the pressure accumulatorUnder the condition of the upward direction, the energy consumption of the

VVVF hydraulic elevator with the pressure accumulator is calculated using formulae(11), (13) and (15), and calculation results of the energy consumption is shown inTable 2.

Under the condition of the upward direction, the energy consumption of theVVVF hydraulic elevator with the pressure accumulator is calculated using formulae(12), (14) and (16), and calculation results of the energy consumption is shown inTable 2.

6.6. Total efficiency of the system

6.6.1. System of the VVVF hydraulic elevator without the pressure accumulator

Under the condition of the upward direction, the efficiency of the VVVF hydrau-lic elevator without the pressure accumulator is given as

gWU PLWU

PmWU17

The data of Table 1 and formula (17) are used to calculate the efficiency of the sys-tem without the pressure accumulator.

Under the condition of the downward direction, the efficiency of the VVVFhydraulic elevator without the pressure accumulator is given as

gWD PfWD

PLWD18

Table 2Energy consumption of the VVVF hydraulic elevator with the pressure accumulator

Motion direction Calculation results of energy consumption (kJ)Upward PLAU = 92 PmAU = 58 PAmU = 103Downward PLAD = 93 PmAD = 71 PApD = 98

Table 1Energy consumption of the VVVF hydraulic elevator without the pressure accumulator

Motion direction Calculation results of energy consumption (kJ)

Upward PLWU = 89 PmWU = 201Downward PLWD = 89 PfWD = 33



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The data of Table 1 and formula (18) are used to calculate the efficiency of the sys-tem without the pressure accumulator.

6.6.2. System of the VVVF hydraulic elevator with the pressure accumulatorUnder the condition of the upward direction, the system efficiency of the VVVF

hydraulic elevator with the pressure accumulator means that the energy consump-tion (PLAU) of load is divided by the sum of the accumulator energy (PAmU) andthe input energy (PmAU) of electrical motor in formula (19).

gAU PLAU

PmAU PAmU19

Under the condition of the upward direction, the data ofTable 2 and formula (19)are used to calculate the system efficiency with the pressure accumulator.

Under the condition of the downward direction, the system efficiency of theVVVF hydraulic elevator with the pressure accumulator means that the accumulatorenergy (PApD) is divided by the sum of the energy consumption (PLAD) of load andthe input energy (PmAD) of electrical motor in formula (20).

gAD PApD

PmAD PLAD20

Under the condition of the downward direction, the data of Table 2 and formula(20) are used to calculate the system efficiency with the pressure accumulator.

The comparisons of total efficiency are shown in Table 3. It is obvious that the

efficiency of the VVVF hydraulic elevator with the pressure accumulator is muchhigher than that of the VVVF hydraulic elevator without the pressure accumulator.

In Table 3, why the VVVF hydraulic elevator is more efficient when it ascendswithout the pressure accumulator?

Formulae (17) and (18) are used to respectively calculate the efficiencies of the sys-tem without the pressure accumulator under the conditions of the upward anddownward direction. When the elevator ascends, the inverter absorbs electrical en-ergy from power supply and directly drives the electrical motor to work. The electri-cal motor drives hydraulic pump to enable the elevator to ascend, and the electricalmotor works at the state of electromotion in the process of ascending. When theelevator descends, the gravity of cabin itself is the power supply for the elevatorsdescending. Hydraulic pumpmotor is working at the state of hydraulic motor,and also the electrical motor becomes electrical generator. The electrical energy gen-erated by the electrical generator is feed back to power supply through inverter addi-tional devices, such as electric reactor and filter, so the lose of the electrical energy is

Table 3Comparisons of total efficiency

Motion

direction

VVVF hydraulic elevator

without the pressure accumulator (%)

VVVF hydraulic elevator with

the pressure accumulator (%)Upward 44 57Downward 37 60



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higher, on the other hand, the electrical generator works at the state of lower effi-ciency, therefore the VVVF hydraulic elevator is more efficient when it ascends with-out the pressure accumulator.

In Table 3, why the VVVF hydraulic elevator is more efficient when it descendswith the pressure accumulator?

Formulae (19) and (20) are used to respectively calculate the efficiencies of the sys-tem with the pressure accumulator under the conditions of the upward and down-ward direction. When the elevator ascends, the energy of both the electrical motorand the pressure accumulator drives the elevator to ascend. When the elevator des-cends, the potential energy of cabin itself is directly stored in the pressure accumu-lator in the form of hydraulic energy without through inverter additional devices,such as electric reactor and filter. In the system with the pressure accumulator, en-ergy transformation is carried out between the pressure accumulator and the cabin,

and the energy lose is less in the process of descending. Therefore the VVVF hydrau-lic elevator is more efficient when it descends with the pressure accumulator.

7. Conclusions

(1) The speed control system of the VVVF hydraulic elevator with the pressureaccumulator has been proposed, which is a kind of innovation energy-savinghydraulic elevator.

(2) Experiments of speed control of the VVVF hydraulic elevator with the pressureaccumulator show that although fluctuating phenomenon exists in the pressureof system, the practical running curve of elevator is almost the same as thedesired running curve of the elevator, and it still runs steadily.

(3) The experimental results show that in the downward direction, the efficiency ofthe VVVF hydraulic elevator with the pressure accumulator is 60%, and theefficiency of hydraulic elevator without the pressure accumulator is only37%, and the efficiency of VVVF hydraulic elevator with the pressure accumu-lator is much higher than that of VVVF hydraulic elevator without the pressureaccumulator. Also hydraulic system with the pressure accumulator is applied

to the other vertical lifting mechanical systems.

Acknowledgement

This work was supported by the National Natural Science Foundation of China.

References

[1] Tanaka H. Fluid power control technology-present and near future. JSME (Jpn Soc Mech Eng) Int J,Ser C (Dynam, Control, Robotics, Des Manuf) 1994;37(4):62937.



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[2] Edge KA. The control of fluid power systems-responding to the challenges, In: Proc of the Institut ofMech Eng, Part I: J Syst Control Eng 1997;211(2):91110.

[3] Backe W. Research and development in fluid power technology. In: Proc of First FPNIPhD Sympos.Hamburg, September 1720, 2000; Hamburg, Germany, 921.

[4] Yao B, DeBoer C. Energy-saving adaptive robust motion control of single-rod hydraulic cylinders withprogrammable valves. In: Proc of the 2002 Am Control Conf. 2002;6. p. 481924.

[5] Yao B, Liu S. Energy-saving control of hydraulic systems with novel programmable valves. In: Proc ofthe 4th World Congress on Intell Control and Automation. 2002;4. p. 321923.

[6] Helduser S. Electrichydrostatic drivean innovative energy-saving power and motion control system.In: Proc of the Institut of Mech Eng. Part I: J Syst Control Eng 1999;213(5):42737.

[7] Li K, Mannan MA, Xu M, Xiao Z. Electro-hydraulic proportional control of twin-cylinder hydraulicelevators. Control Eng Practice 2001;9(4):36773.

[8] Yang HY, Yang J, Xu B. Computational simulation and experimental research on speed control ofVVVF hydraulic elevator. Control Eng Practice 2004;12(5):5638.

[9] Xu, B. Research on inverter controlled hydraulic energy-saving system using pressure accumulator.

Dissertation of Doctor Degree, Zhejiang University, Hangzhou, Zhejiang, China, 2001.


Documents

Xu System Efficiency