Advanced Control Concepts Suitable for Energy Efﬁcient Hydraulic

Systems

Tadej Ta

sner

1,2

, Vito Ti

and Darko Lovrec

HAWE Hidravlika d.o.o., Petrov

ce, Slovenia

Faculty of Mechanical Engineering, University of Maribor, Maribor, Slovenia

Keywords:

Hydraulics, Simulation, Control, Asynchronous Motor, Variable-frequency Drive Controller, Constant Pump,

Variable Displacement Pump, Hydraulic Tubing.

Abstract:

Today there are ever-increasing demands for more efﬁcient hydraulic drive technology in terms of reducing

energy consumption, increasing reliability plus robustness, and for minimising the maintenance interventions

on the drive. In addition, the requirements and directives on the reduction of the noise, and development

tendencies in the direction of environmentally user-friendly drives, are leading to an ever-increasing usage of

the electro-hydraulic drive technology. There are two main concepts for converting electrical into hydraulic

energy: constant speed motor coupled with variable displacement pump and variable speed motor coupled with

constant pump. This article presents completed PhD tasks including modelling and simulation of both of the

two concepts along with a new concept of variable speed motor coupled with constant pump. All the concepts

are compared in dynamics and efﬁciency based on the simulation results. The expected results of the PhD are

a newly-synthesized SIMO controller for ’bi-variable’ control, that will be able to control hydraulic systems

in order to operate within the areas of maximum efﬁciency, highest dynamics or a compromise between these

two. A further contribution to the hydraulic system developers’ community will be an experimentally proven

mathematical model for the simulations of different hydraulic drive concepts. Such a model may be used for

optimising the energy-efﬁciencies of existing and new hydraulic machinery, as well as efﬁciency prediction

when building new hydraulic systems. Moreover it may also be used for determining the most suitable drive

concept for a hydraulic system with a predeﬁned operating cycle.

1 RESEARCH PROBLEM

Today there are ever-increasing demands for more ef-

ﬁcient hydraulic drive technology in terms of reduc-

ing energy consumption, increasing reliability plus

robustness, and for minimising the maintenance in-

terventions on the drive. In addition, the requirements

and directives on the reduction of the noise, and devel-

opment tendencies in the direction of environmentally

user-friendly drives, are leading to an ever-increasing

usage of the electro-hydraulic drive technology. This

has come to light especially in areas where higher

energy density is required, for example, on modern

milling and forming machines as well as in the seg-

ment of machinery and equipment that operates con-

tinuously, autonomously, at remote sites and without

the presence or supervision of the maintenance staff.

Such applications include:

• highly loaded machining centres

• wind-farms (especially those installed in such as

waterside bay areas)

• mobile machines (excavators, cranes, etc.)

Hydraulics are mainly used in systems where big

forces and power are required for normal operations.

High-power requirements reﬂects in high-energy con-

sumption. Even the slightest (at the percentage level)

optimisations regarding the energy efﬁciency of such

machines could result in signiﬁcant savings over long

operating periods. Therefore, improved efﬁciency

and reduced energy consumption are two of the main

goals during modern electrohydraulic drive systems

designing.

2 STATE OF THE ART

Most of the efﬁciency increases could be achieved

Tašner T., Ti

c V. and Lovrec D. (2013).

Advanced Control Concepts Suitable for Energy Efﬁcient Hydraulic Systems.

In Doctoral Consortium, pages 3-11

DOI: 10.5220/0004637000030011

 SciTePress

during the electrical to hydraulic energy conversion

stage. In general hydraulic energy can be controlled

in two main ways:

• the throttling principle

by throttling on the directional valve

• the volumetric principle

by adjusting the pump displacement volume

The throttling principle displays good dynamic be-

haviour, but its energy losses are substantial. The

volumetric principle is more energy-friendlier but has

even worse dynamic response (Majumdar, 2000). The

volumetric principle is mostly used due to its better

efﬁciency.

There are two more commonly used drive con-

cepts for the volumetric adjustment of hydraulic en-

ergy:

• the direct concept

by adjusting the displacement of a variable

displacement pump

• the indirect concept

by adjusting the rotational speed of a con-

stant displacement pump

Most machines within the ﬁeld of hydraulic drive

technology are still using the classic drive con-

cept—variable displacement pump driven at constant

speeds. Desiring greater robustness and lowering the

price of hydraulic drives over recent years, as well as

lower prices for variable-frequency drive controllers,

has led to the more and more popularity for using

speed-controlled constant pumps. However, such a

concept can not meet the dynamic requirements of

the classic drive concept (Lovrec and Ulaga, 2007;

Lovrec et al., 2005). Therefore the question arises as

to ’Which drive concept would be more efﬁcient for a

particular hydraulic application?’.

Table 1 shows a comparison between both drive

concepts: variable pump and constant motor (C1),

as well as constant pump and variable speed drive

(C2). C2 has asserted itself mainly due to good efﬁ-

ciency and a wide operating range (Xu et al., 2010). It

also excels at lower operating costs and consecutively

smaller affects on the environment. Lower operating

costs and higher efﬁciency have come mainly from

a more efﬁcient connection of the motor to the elec-

trical grid—via a variable-frequency drive controller

(Ferreira et al., 2011). However, C2 also has one ma-

jor drawback a slow system response that occurs due

to rotating parts’ moment of inertia (motor rotor and

rotary parts of the pump). The response of C1 is up to

5 times faster than the response of C2 (Lovrec et al.,

2005), but both responses are good enough for the

most hydraulic applications (Lovrec et al., 2009).

Table 1: Comparisons between both the more-commonly

used hydraulic drive concepts.

Concept 1

(C1):

Concept 2

(C2):

Frequency

Inverter

Asynchronous

Motor

Asynchronous

Motor

Variable Axial

Piston Pump

Constant Gear

Pump

efﬁciency lower higher

reliability high high

operating

costs

higher lower

system

dynamics

higher

4.4

lower

purchase

price

higher lower

If both concepts (C1 and C2) were to be com-

bined, then we would obtain increased dynamics due

to the variable displacement pump (Song et al., 2008)

and increased efﬁciency due to the variable-frequency

drive controller (Ferreira et al., 2011). Such a com-

bined drive concept (Figure 1), as well as the effect of

different motor speeds and different pump displace-

ments on the efﬁciency and dynamics of the system,

yet this is rarely mentioned in literature.

Figure 1: Combined drive concept (C3) - variable speed

asynchronous motor coupled with a variable displacement

pump.

3 OUTLINE OF THE

OBJECTIVES

Based on the experience gained during the initial

study of the dynamic behaviour and implementation

of appropriate control strategies of both drive con-

cepts, this new drive concept (a combination of vari-

able speed drive and variable displacement hydraulic

SIMULTECH2013-DoctoralConsortium

pump) needs to be explored in more detail. In order

to achieve this objective both existing concepts will

have to be studied on the basis of simulation and ex-

perimental analysis. The presented study will focus

on both the selection and implementation of an ap-

propriate control concept for this new drive concept,

as well as the comparisons between the efﬁciencies

for all three drive concepts.

The results obtained on the basis of these stud-

ies will be transferred to the proposed drive concept.

Firstly, a new solution for the control concept will

have to be found, because multiple physical quantities

have to be monitored and changed at the same time:

• swashplate angle of the variable axial piston pump

• rotational speed of the motor

As both quantities directly effect oil pressure within

the hydraulic circuit, an appropriate control strategy

for such Single Input Multiple Output (SIMO) sys-

tem will have to be synthesised. Such a control strat-

egy then raises some further questions. Which quan-

tity should be changed and how much, in order to

maximise the efﬁciency and/or dynamics of the drive?

What are the efﬁciency beneﬁts of the new drive con-

cept for the particular operating cycles of different

machines?

4 METHODOLOGY

These research activities have an interdisciplinary na-

ture, since they combine the knowledge of technical

apparatus (speciﬁcations, principle of operations and

performances of the individual components) as well

as classic and modern methods for controlling sys-

tems. The starting point of scientiﬁc activities is a

good knowledge of the individual components, their

speciﬁc structures and modes of operation that is ob-

tained from the manufacturer or provider of com-

ponents. All of those properties are needed when

constructing a mathematical model for simulating all

three drive concepts. It is necessary to evaluate all

the existing drive concepts and compare them using

simulation as well as experimentally.

Appropriate and powerful simulation tools need

to be used for simulation. The existing test site and

equipment to be used for experimental veriﬁcation of

the simulation results will have to be redesigned and

updated in terms of performance, ﬂexibility and ver-

satility. A new system for controlling and monitoring

the test site will have to be designed that includes au-

tomatic set-point and disturbance generation, as well

as data acquisition and archiving.

5 EXPECTED OUTCOME

The expected outcome of this research is to make

some original contributions to the development of this

scientiﬁc discipline. The ﬁrst scientiﬁc contribution

of the PhD work is a mathematical model for evalu-

ation of the dynamics and efﬁciency of different hy-

draulic drive concepts that can be veriﬁed on a test-

site. A further contribution is the design of an appro-

priate control strategy for ’bi-variable’ (variable dis-

placement, variable speed) hydraulic drive units that

will be based on modern control and decision-making

concepts.

6 STAGE OF THE RESEARCH

Most of the theoretical part of the work has been al-

ready done, meaning that the simulation model al-

ready consists of the following mathematical models:

• variable-frequency drive controller,

• asynchronous motor,

• variable displacement axial piston pump,

• ﬁxed displacement internal gear pump,

• and hydraulic tubing

All the simulations regarding the efﬁciencies of dif-

ferent drive concepts have been run. Some results

have already been experimentally veriﬁed on a test-

site.

Therefore the used simulation model, including

the simulation results, are presented in this article.

7 SIMULATION MODEL

The simulation model of C1 consists of an asyn-

chronous motor directly connected to grid, a vari-

able displacement axial piston pump, and hydraulic

tubing. The simulation model of C2 consists of

a variable-frequency drive controller with an asyn-

chronous motor connected to it, a ﬁxed displacement

internal gear pump, and hydraulic tubing. The simu-

lation model of C3 consists of a variable-frequency

drive controller with an asynchronous motor con-

nected to it, a variable displacement axial piston

pump, and hydraulic tubing.

The asynchronous motor is coupled to the pump

that pumps the hydraulic ﬂuid from a hydraulic tank

through a long length of hydraulic tubing, and back

to the tank. The ﬂow through the tubing causes a

pressure drop that is measured directly after the pump

using a pressure sensor. The following subsection

AdvancedControlConceptsSuitableforEnergyEfficientHydraulicSystems

presents the models of all the components used in

simulation.

7.1 Asynchronous Motor Model

The more simpliﬁed model of the three-phase asyn-

chronous motor consists of two pairs of magnetically-

coupled symmetrical three-phase windings. Both of

the three-phase windings (stator winding and rotor

winding) are identical. The same model also ap-

plies for a squirrel-cage rotor, where currents start

ﬂowing due to electro-magnetic induction. The more

commonly used model for the dynamic simulation of

asynchronous motors is based on the so-called ’T’

equivalent circuit (Figure 2). Such a model is used for

static and dynamic simulations, although it neglects

core losses (saturation and eddy current losses). (Diaz

et al., 2009) The following equations can be obtained

Figure 2: Electrical representation of the ’T’ equivalent cir-

cuit of an asynchronous motor.

from the ’T’ equivalent circuit for a squirrel-cage ro-

tor:

= R

+ L

0 =

= R

+ L

(1)

where

I are vectors of voltage/current phasors for

each phase, R winding resistances and L inductances,

where subscript ’r’ denotes rotor, ’s’ stator and ’M’

mutual. The electromechanical torque can be written

as:

= T

= J

dΩ

+ b · Ω + T

(2)

where Ω is the mechanical rotational frequency of the

rotor, b the viscous friction coefﬁcient and T

the load

torque. (Delaleau et al., 2001)

All the necessary parameters of the asynchronous

motor needed for the simulation model were calcu-

lated from the measurements’ results from the locked

rotor and no-load tests.

7.2 Variable-frequency Drive

Controller Model

The variable-frequency drive controller is a device

that can change the rotational frequency or torque

of an electric motor by modifying the frequency and

amplitude of the motor’s supply voltage. A typical

variable-frequency drive controller consists of (Fig-

ure 3):

• a rectiﬁer,

rectiﬁes input AC voltage

• DC link,

energy storage

• an inverter,

converts DC to AC

• and a control circuit.

measures motor currents and controls the in-

verter

Figure 3: Block diagram of a variable-frequency drive con-

troller.

7.2.1 Rectiﬁer Model

The more commonly used rectiﬁer type in variable-

frequency drive controllers is a full-wave rectiﬁer

with an LC ﬁlter. Such a rectiﬁer consist of three pairs

of rectiﬁer diodes and a ﬁlter inductor, which charge

the DC link capacitor. Electrical circuit of such rec-

tiﬁer is shown in Figure 4. Assuming that the LC ﬁl-

ter is ideal and that the voltage drop on the diodes is

constant, the rectiﬁer losses can be expressed as (3),

where U

is the diode forward voltage. The factor 2

in the equation represents current ﬂows through one

lower and one upper diode.

RECT

= 2 · I

·U

(3)

Figure 4: Three phase full wave rectiﬁer with LC ﬁlter.

SIMULTECH2013-DoctoralConsortium

7.2.2 DC Link Model

An ideal DC link capacitor is used in our variable-

frequency drive controller.

7.2.3 Inverter Model

Most of the inverters are controlled based on the

principle of space vector modulation. A three-phase

vector Pulse Width Modulation (PWM) inverter re-

quires three transistor bridges. Each bridge consists

of two transistors operating in the switching region.

Such an inverter can drive any three-phase load, with

or without the neutral line. Figure 5 shows such

an inverter, where the transistors are represented as

switches. The transistors used in the inverters are

either Isolated Gate Bipolar Transistors (IGBTs) or

Metal Oxide Semiconductior Field Effect Transistors

(MOSFETs) and are controlled by a microcontroller.

A simpliﬁed loss model for switching and conductive

losses of MOSFETs will be used (Shen et al., 2006),

because our inverter uses MOSFETs. The energy loss

in one switch of both transistors within a transistor

bridge, can be expressed as (4). Where all the data

are characteristics of the MOSFET except U and I, are

dependent on the inverter load, and represent the volt-

age on the MOSFET and the current ﬂowing through

the MOSFET, respectively.

OFF

)

(4)

The conducting losses for the transistors during their

on time are (4)

= R

DS(on)



+ i



(5)

Figure 5: Three phase bridge inverter.

and the total losses are (6)

INV

= P

(6)

where f

is the inverter switching frequency.

7.2.4 Control Circuit Model

Most variable-frequency drive controllers control the

drives using the Sensorless Vector Control (SVC)

principle. SVC needs an inverse dynamic model of

the asynchronous motor in dq coordinates for it to

function. Synthesis of such a model is beyond the

scope of this article. The basic parts of its synthesis

are presented in (Stekl, 2007). A simpliﬁed block di-

agram of the control circuit is shown in Figure 6, its

main blocks are:

• rotor ﬂux estimator,

rotor ﬂux is needed for decoupling

• decoupling,

decouples d and q voltage components so

each can be changed without effecting the other

• two current controllers,

control magnetising (d) and torque (q) cur-

rent components by changing their voltages

• speed controller,

controls motor rotational speed by changing

the torque current component

• and load compensation.

detects the load and increases the required

torque current accordingly, in order to maintain

the desired rotational speed

Figure 6: Control circuit block diagram.

7.3 Hydraulic Pump Models

Gear and an axial piston pump were used for the

conversion of mechanical energy into hydraulic en-

ergy. Gear pumps have constant displacement vol-

ume, whereas axial piston pumps may also be ad-

justable. The ﬂow through them can be changed by

adjusting the swash-plate angle.

AdvancedControlConceptsSuitableforEnergyEfficientHydraulicSystems

Wilson (Wilson, 1949) started modelling hy-

draulic pumps at the end of the ﬁrst half of the 20th

century. He designed a static model that takes into ac-

count volumetric and frictional losses of pumps. His

model was later improved by some authors, their work

was summed-up by Rydberg (Rydberg, 2009). How-

ever, these models are only useful for the already-

made and tested pumps, because their coefﬁcients

are obtained from measurements and experiments on

the stations. (Jeong and Kim, 2007) Our models are

based on Wilson’s model and use certain modiﬁca-

tions mentioned by Rydberg.

7.3.1 Fixed Displacement Internal Gear Pump

Model

The ﬂow Q of the hydraulic ﬂuid through the gear

pump is the proportional to the rotational speed n

of the pump. Both quantities are connected by the

pump’s displacement V

, which is a characteristic

property of each pump. At higher pressures the

pump’s ﬂow slowly starts to drop, due to the com-

pressibility of the ﬂuid and the greater leakage losses.

These losses are covered under the concept of the vol-

umetric efﬁciency of the pumps, which depends on

the pressure difference ∆p that the pump must create.

Such losses can easily be presented as a parallel hy-

draulic resistance R

of the ideal pump (an analogy

with Norton’s theorem in electrical engineering). (7)

Q = n ·V

−

∆p

(7)

The pump’s operating torque is proportional to the

pressure difference that the pump creates, due to the

law of energy conservation. The torque T must, in ad-

dition to creating pressure difference, overcome those

losses that occur due to lubrication of the pump and

friction between the moving parts of the pump b. (8)

T = ∆p ·V

+ n · b (8)

7.3.2 Variable Displacement Axial Pump Model

Similarly as for the constant gear pump, an equation

can also be written for the variable displacement axial

piston pump. The ﬂow of the pump is affected by the

pump’s displacement setting α, therefore we obtain

(9) from (7).

Q = n · α ·V

−

∆p

(9)

The pump’s displacement setting has a similar ef-

fect on the required operating torque. The losses ap-

pear due to the lubrication and friction of the rotating

parts b, but additional losses of the friction between

piston and walls C must also be taken into account.

(10)

T = ∆p · α ·V

+ ω · α · b + ω ·

tanα

max

·C (10)

7.4 Hydraulic Tubing Model

The dynamic behaviour of the ﬂuid within the

pipeline can be modelled in several different ways.

The most exact model is based on the Navier-Stokes

equations and the law of mass conservation, which re-

sults in a system of partial differential equations that

are too much time consuming for such simulations.

Such an exact model of a hydraulic pipeline was

unnecessary, therefore a more appropriate—discrete

model was chosen, also known as a model with con-

centrated parameters. The discrete model is simi-

lar to electrical circuits used by electrical engineers,

where the properties of a circuit are represented by

resistance, capacitance and inductance. In hydraulics

the properties of a pipeline system are hydraulic re-

sistance R

(pressure drop in a tube due to ﬂow),

hydraulic capacitance C

(pressure drop in a tube

due to tube volume increase/decrease) and hydraulic

inductance L

(pressure drop due to ﬂuid accelera-

tion/deceleration). The analogy between electronics

and hydraulics is presented in Table 2.(Ta

sner and

Lovrec, 2011)

Table 2: Electrical-hydraulic analogy.

Electrical

Symbol

Electrical

Equation

Hydraulic

Equation

U = R · I p = R

· Q

p = L ·

p = L

U =

Idt

p =

Qdt

The hydraulic pipeline system split into n seg-

ments can be represented using the electrical symbols

as shown in Figure 7. Each segment represents part

of a pipeline with a length of l/n, where l is the total

length of the pipeline. The number of segments also

equals the number of possible pressure measurement

points (For example, if the tube is modelled as one

segment, the pressure in the middle of the tube cannot

be calculated.)

The transfer function of such a segment can be ex-

pressed as a second-order ordinary differential equa-

SIMULTECH2013-DoctoralConsortium

Figure 7: Hydraulic tubing represented using electrical

symbols.

tion within the Laplace frequency domain (11).

· s + R

· s

· s + 1

(11)

8 SIMULATION RESULTS

Simulations were performed on different combina-

tions of simulation models, as described in the previ-

ous section, using MATLAB/Simulink software. The

same hydraulic tubing was used as a load when simu-

lating all three concepts. Pressure control was realised

using a PID controller using a clamping anti-windup

method for the integral part.

Figure 8: Control block diagram of C1 and C1*.

• In C1 the pressure is controlled by changing the

axial piston pump’s displacement, as shown in

Figure 8. The asynchronous motor (ASM) was

connected directly to power-grid (C1), and via the

variable frequency drive controller, set to a con-

stant rotational speed of 1500 min

−1

(C1*). C1*

was just added to include the variable-frequency

drive controller’s losses to C1.

• In C2 the pressure is controlled by changing mo-

tor’s rotational speed that drives the constant gear

pump, as shown in Figure 9.

Figure 9: Control block diagram of C2.

• In C3 the pressure is controlled by two parallel

PID controllers that change the motor’s rotational

speed and the axial piston pump’s displacement,

as shown in Figure 10.

Figure 10: Control strategy of C3.

The pressure set-point was changed according to

a combined cycle (sine, ramp and step). The pres-

sure responses of the different drive concepts were ob-

served and compared according to dynamics and ef-

ﬁciency. The simulation results are presented in Fig-

ures 11- 12, and in Table 3. The upper part of the Ta-

ble 3 shows the efﬁciency comparisons. The average

efﬁciency is calculated as a quotient of the produced

hydraulic energy and the required electrical energy.

A ﬁrst look at the efﬁciencies reveals that C1 had the

highest efﬁciency, due to the fact that a variable fre-

quency drive controller was not used in C1. The new

combined concept (C3) was the most efﬁcient from

amongst all the concepts that use variable-frequency

drive controllers. If we had wanted to compare oper-

ating cycle costs, apparent power would have to have

been compared, as companies pay for both the real

power [W] and the reactive power [VAr]. For most

of the new variable-frequency drive controllers, real

power is almost equal to apparent power, due to built-

in power-factor corrections (their cosφ ≈ 1, as reac-

tive power ﬂows only between the motor and the con-

troller).

The bottom part of the Table 3 compares all the

concepts according to control performances. Settling-

times in response to step from 50-150 bar were calcu-

lated - a close-up of the step responses is shown in

Figure 12. Settiling-time is the time when the pres-

sure within the pipeline settled within 1 % of the pres-

sure set-point. Moreover, the Root Mean Square Er-

ror (RMSE) was calculated for the whole cycle and

for the tracking part (ramp and sine). It can be seen

that the C1 had the best dynamics and lowest overall

RMSE. C2 had the worst RMSE as well as the lowest

dynamics with settling-time 6 times slower than C1.

This was mostly due to the moment of inertia of the

motor rotor and the rotating parts of the pump, as the

motor must accelerate and decelerate according to the

pressure set-point changes. Although C1 had the best

dynamics, C3 tracked the pressure set-point more ac-

curately.

AdvancedControlConceptsSuitableforEnergyEfficientHydraulicSystems

Table 3: A comparison between different drive concepts’

simulation results.

C1 C1* C2 C3

average

efﬁciency

72 % 60 % 65 % 67 %

energy

required

[kJ]

31.6 33.8 30.9 32.2

energy

produced

[kJ]

22.7 20.1 20.2 21.7

RMSE

[bar]

4.92 6.40 12.9 7.16

tracking

RMSE

[bar]

0.231 0.227 5.95 0.178

settling

time [ms]

92 95 570 176

Figure 11: Responses of the different drive concepts to the

combined cycle.

9 CONCLUSIONS

The simulation results showed that all three concepts

are useful in the ﬁeld of hydraulic drives. Although

C2 had relatively slow dynamics, it can be used for

processes where high dynamics is not critical. While

C1 is the oldest concept, it still has the best efﬁciency

- but we must also consider the reactive power of the

asynchronous motor when it is powered directly from

the grid. The newly-proposed concept (C3) had a very

good tracking performance of slower changes of the

pressure set-point, moreover it also had the best efﬁ-

ciency of all the variable-frequency drive controlled

Figure 12: Step response close-up.

concepts. Its dynamics could be improved by imple-

menting a better controller for the ’bi-variable’ pres-

sure control. The controller could use a look-up ta-

ble for efﬁciencies and set the swashplate angle and

motor rotational speed to such value that maximum

efﬁciency is achieved.

10 FURTHER WORK

Further steps in pursuing the PhD are:

• experimental veriﬁcation of efﬁciencies for each

modelled component on the test site,

• synthesis of a new SIMO controller for ’bi-

variable’ pressure control (max dynamics or max

efﬁciency control),

• tuning and testing of the new controller within a

simulation environment,

• experimental evaluation of the tuned SIMO con-

troller and

• experimental evaluation of the efﬁciencies of all

three concepts.

11 EXPECTED OUTCOME

The expected results of the PhD are a newly-

synthesized SIMO controller for ’bi-variable’ con-

trol, that will be able to control hydraulic systems in

order to operate within the areas of maximum efﬁ-

ciency, highest dynamics or a compromise between

SIMULTECH2013-DoctoralConsortium

these two. A further contribution to the hydraulic sys-

tem developers’ community will be an experimentally

proven mathematical model for the simulations of dif-

ferent hydraulic drive concepts. Such a model may

be used for optimising the energy-efﬁciencies of ex-

isting and new hydraulic machinery, as well as efﬁ-

ciency prediction when building new hydraulic sys-

tems. Moreover it may also be used for determining

the most suitable drive concept for a hydraulic system

with a predeﬁned operating cycle.

ACKNOWLEDGEMENTS

Operation part ﬁnanced by the European Union,

European Social Fund. Operation implemented in the

framework of the Operational Programme for Human

Resources Development for the Period 2007-2013,

Priority axis 1: Promoting entrepreneurship and

adaptability, Main type of activity 1.1.: Experts and

researchers for competitive enterprises.

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AdvancedControlConceptsSuitableforEnergyEfficientHydraulicSystems