*Corr. Author’s Address: Lodz University of Technology, Institute of Machine Tools and Production Engineering, 90-924 Łódź, Poland, andrzej.kosucki@p.lodz.pl 599
Strojniški vestnik - Journal of Mechanical Engineering 67(2021)11, 599-610 Received for review: 2021-07-09
© 2021 Journal of Mechanical Engineering. All rights reserved.  Received revised form: 2021-09-30
DOI:10.5545/sv-jme.2021.7320 Original Scientific Paper Accepted for publication: 2021-10-20
Electro-Hydraulic Drive of the Variable Ratio Lifting Device 
under Active Load
Kosucki, A. – Stawiński, Ł. – Morawiec, A. – Goszczak, J.
Andrzej Kosucki
1
 – Łukasz Stawiński
1
 – Adrian Morawiec
1
 – Jarosław Goszczak
2
1
Lodz University of Technology, Institute of Machine Tools and Production Engineering, Poland 
2
Lodz University of Technology, Department of Vehicles and Fundamentals of Machine Design, Poland
Hydraulic systems fed by fixed displacement pumps driven by frequency-controlled electric motors can replace conventional throttling systems 
due to their ability to control the speed of hydraulic cylinders regardless of the value and direction of the load. These systems can improve the 
energy efficiency of the drive or even provide the possibility of energy recuperation during lowering. This paper presents experimental studies 
of the new drive system with volumetric control of the speed of the lifted/lowered payload using the example of a scissor lift. The system uses 
a reversible gear pump driven by an asynchronous motor fed by a frequency inverter operating in field-oriented control mode. Comparative 
studies of the mapping of the assumed speed of the hydraulic cylinder and platform are presented, as well as studies of the influence of the 
load change on the speed and positioning of the mechanism driven by the open-loop controlled system.
Keywords: hydraulic drives, speed controlled pump, variable-ratio device, lifting system
Highlights
• 	 High-speed accuracy is achieved regardless of the load direction.
• 	 The structure of the drive system is simplified compared to classic solutions.
• 	 The system makes it possible to use volumetric control even for low-power and low-efficiency systems.
• 	 Improving the energy efficiency of the drive is possible.
0  INTRODUCTION
Hydraulic excavators, elevators, wheel loaders, 
various forklifts, scissor lifts, and other mobile and 
stationary machinery use hydraulic lifting systems 
with hydraulic cylinders to drive the payload 
with throttling control during payload lowering. 
The lowering phase is usually caused by gravity. 
Generally, this requires the use of appropriate valves 
or complex control systems to protect the load against 
uncontrolled falling. The most common solution is 
to use a throttle valve. The typical hydrostatic lifting 
system, presented in Fig. 1, consists of a hydraulic 
power supply unit equipped with an oil tank assembly 
(3), the motor (1), the oil pump (2), and a set of valves, 
such as the pressure relief valve (4), the directional 
valve (5), the one-way throttling valve (6) and the 
rupture valves (7) screwed into the inlets of hydraulic 
cylinders (8). 
The one-way throttling valve during the lowering 
payload operates as an adjustable choke in which the 
oil flow rate depends on a pressure drop between the 
inlet and outlet Dp
s
, according to Eq. (1):
 QA c
p
DD D
s
 
2

, (1)
where A
D
 is the cross-sectional area of the valve 
orifice, c
D
  the coefficient of flow losses, and ρ the 
density of the hydraulic liquid.
Usually, the throttle valve is set to the defined 
value of the cross-sectional area of the orifice. When 
the hydraulic cylinder is loaded with a constant force, 
this results in a constant pressure under the piston and 
a constant lowering speed, according to Eq. (2): 
 v
dx
dt
Q
d
s
s D
s

 
2
4
, (2)
where x
s
 is the length of the hydraulic cylinder, and d
s
 
the diameter of the hydraulic cylinder.
However, many mechanisms act on hydraulic 
cylinders with a variable force during their movement, 
which could be caused by a load change or a variable 
ratio of driven mechanisms. The changing force acting 
on the hydraulic cylinder causes the variable pressure 
under the piston. The higher the pressure, the greater 
the flow rate through the throttle valve, and thus the 
increase of the payload lowering speed. It happens 
when the mechanism ratio changes during the duty 
cycle (together with the extension/retraction of the 
hydraulic cylinder). Then the speed of the piston 
rod changes with the change of mechanism ratio, as 
shown in Fig. 2. 
This type of drive has many disadvantages, such 
as heating the oil on the throttle valve during the 
lowering phase, resulting in the need to dissipate the 
potential energy. It could be then necessary to use oil 
coolers, which increases the cost of the system. The 

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dynamic overloads of the cylinder and mechanism 
appear caused by a directional valve that is rapidly 
opened (or closed) by a solenoid, which can lead to 
premature wear of the machine. The speed of payload 
lowering using the throttle valve is less controllable.
Fig. 1.  Schematic of the conventional hydraulic lifting system
Fig. 2. The exemplary relationship between the speed of the piston 
rod v
s
, the speed of the platform v
p
, and its load F during lowering
In addition to systems with a throttle valve, other 
solutions are used in hydraulic systems to control 
hydraulic cylinders loaded with the active force. 
The review of the controllability of such systems 
is described in [1]. These systems use variable 
displacement pumps, throttle valves, or proportional 
and servo valves. There are also special braking 
valves used in hydrostatic systems driving hydraulic 
cylinders with an active load, called counterbalance 
valves, described in [2]. In [3], variable hydraulic 
cylinder load (from passive to active) during one full 
cycle of motion was discussed. The author dealt with 
the subject of piston rod braking with the use of, among 
others, controlled check valves and various types of 
counterbalance valves. The principle of operation and 
the benefits of counterbalance valves, which are load 
holding, load control, and load safety, described in [4]. 
Incorrect application can lead to negative pressure or 
stoppage of operation of the device due to insufficient 
pressure difference. Problems that may occur in 
systems with counterbalance valves are described in 
[5]. The use of the right valve depends on the needs, 
which is why it is so important to know about the 
capabilities of a given element. A detailed division 
of counterbalance valves and their advantages and 
disadvantages are described in [6] and [7].
Counterbalance valves are used to control the 
movement of the actuator, usually with an active or 
variable load. Authors in [8] describe an approach 
that shifts the task of throttling the return flow from 
the counterbalance valve to the directional valve. The 
counterbalance valve can also be used to start and 
stop rotating parts smoothly. The controlled motion of 
hydraulically actuated transmissions characterized by 
large inertias, backlash, and several parallel-coupled 
gearmotors is described in [9]. The analysis of the 
operation of the counterbalance valves was also tested 
on the basis of simulation models. The principle of 
operation of a pressure valve with two control signals 
is described in [10]. Based on the counterbalance 
valve model, the author [11] attempted to analyse 
the influence of the brake valve setting on the valve 
response. The counterbalance valve operation 
model with its various settings was verified in the 
crane’s drive system [12]. The use of counterbalance 
valves can also reduce the energy consumption of 
the systems. In [13], a closed-circuit system with 
hydraulic motors and counterbalance valves for 
supplying an electro-hydraulic actuator is described. 
The use of such systems yields a 50 % lower energy 
consumption. The possibilities of reducing pressure 
peaks and drops, described in [14], lead to a reduction 
of energy consumption by up to 80 %. The use of a 
counterbalance valve in a variable load system in a 
truck crane [15] reduces the oscillation by up to 40 %, 
which increases the efficiency and extends the service 
life of the machine. During the operation of such 

Strojniški vestnik - Journal of Mechanical Engineering 67(2021)11, 599-610
601 Electro-Hydraulic Drive of the Variable Ratio Lifting Device under Active Load
systems, the problem may be the need to generate 
additional pressure for pump operation and oil heating 
because of the friction caused by flowing through 
these valves. In addition, the settings of such valves 
must be adapted to the acting forces.
The solution that eliminates the problem of load 
change is the load-sensing system. A comparative 
study of scissor lift drives carried out based on digital 
simulation technology of the quantitative pump 
hydraulic system and the load-sensing hydraulic 
system is presented in [16]. The author found that the 
load-sensing hydraulic system is more energy-saving, 
stable, and prevents interference than the conventional 
one. However, the system is sensitive to setting the 
pressure difference on the valve, which was set after 
the batch analysis. Moreover, it is still an expensive 
solution.
A volumetric control can be used to control 
the speed of the hydraulic cylinder. A variable 
displacement pump can be used, but preferably in 
systems with higher power (8 cm³/rev). Another 
method is to use a device that changes the speed of 
the motor driving the fixed displacement pump, called 
a variable speed pump drive (VSPD). The problem of 
maintaining the assumed speed of the motor is solved 
in electric drives by using a frequency inverter (FI), 
which are increasingly used in hydraulic systems 
instead of complex and expensive systems with 
variable volume pumps.
The implementation of electronics to hydraulic 
systems gives a much greater possibility of controlling 
the movement of actuators and increases energy 
benefits in relation to typical valve solutions. 
Positioning control methods can be based on digital 
pump-motor technology with multiple independent 
outlets connected directly to the cylinder chambers, 
which eliminates the need for valves and the 
generation of energy losses. Described in [17], 
the digital hydraulic power management system 
(DHPMS) is a solution that significantly improves the 
energy efficiency of hydraulic systems [17]. Another 
example to achieve the double goals of high precise 
tracking performance and high energy efficiency, a 
completely new hardware configuration that connects 
a direct driven pump and independent metering valves 
is proposed in [18]. The combination of a pump 
efficiency control system in the form of a variable 
speed drive motor and a proportional valve set offers 
energy advantages over conventional valve systems. 
Both systems operate only in an open-loop controlled 
based on the position tracking of the hydraulic 
cylinder.
In [19], an experimental forklift drive with the 
possibility of energy recovery is presented. However, 
this is not a very popular method of controlling the 
speed of a hydraulic cylinder. Three different ways 
of control, the pressure of the hydrostatic drive using 
variable displacement pump and FI were presented in 
[20]. The authors in [21] presented the possibility of 
controlling the output flow of a hydraulic pump driven 
by a motor with FI. In [22], the authors described the 
use of FI in several hydrostatic systems, including 
the drive of hydraulic cylinders. These systems 
operate based on direction pumps in the following 
configurations: variable displacement pumps co-
operating with a motor supplied by the power network 
or a fixed displacement pump with a motor fed by FI. 
Performances for the same three systems in [23] have 
been discussed, more specifically including efficient 
displacement. 
An interesting combination of methods of 
controlling the speed of an actuator loaded with 
variable force is the research on a self-contained 
electro-hydraulic cylinder (SCC) presented in [24]. 
The system, in the author’s opinion, has the potential 
to replace both conventional hydraulic systems 
and electro-mechanical counterparts, enhancing 
energy efficiency and reducing maintenance. The 
system combines an electric servomotor and a fixed 
displacement pump, the single rod double-acting 
cylinder, and a low-pressure accumulator arranged 
in a closed-circuit configuration. The issue of energy 
recovery in hydrostatic systems with double-acting 
cylinders also appears in [25]. The double-pump 
system, with its displacement balanced with the piston 
area ratio, is used.
Most of the available literature deals with 
a double-acting cylinder, where the control 
possibilities are wider than in the case of a plunger 
cylinder (single-acting), which is presented in this 
article.
The use of the FI in typical drive systems with 
directional flow valves is the simplest and relatively 
inexpensive solution. In [26], the research of the use 
of FI together with a simple flow directional valve 
to control the scissor lift operation was presented. 
The authors described the possibility of lifting the 
system with a variable ratio, with different loads, 
maintaining a constant speed of the working platform. 
Based on the platform position sensor and known 
machine geometry, the FI control function was 
calculated. The generated input functions proportional 
to the assumed speed with proper acceleration and 
deceleration times allowed reducing the vibration of 
the hydraulic cylinder, the entire machine structure, 

Strojniški vestnik - Journal of Mechanical Engineering 67(2021)11, 599-610
602 K o suc ki,	A .	–	S ta wiński,	Ł.	–	Mo ra wiec,	A .	–	Go szczak ,	J.
and the transported payload. However, lowering the 
payload using a scissor lift is a more complex matter 
and requires additional assumptions, which have not 
yet been presented in the publications. Additionally, 
the usage of the proposed system can also improve the 
energy efficiency, as presented in [27].
The proposed new simple electro-hydraulic 
system with a fixed displacement pump driven by an 
electric motor fed by FI in field-oriented mode and a 
simple controller fulfill the main goals of this control 
system, which are:
• ability to shape the speed of the hydraulic cylinder 
(or any actuator) freely, both during lifting and 
lowering;
• maintaining the assumed speed of the hydraulic 
cylinder (or any actuator) independently from the 
value and direction of the load.
The authors developed a simple hydraulic system, 
which, using the FI properties, allows full control of 
the speed of the cylinder or the lift platform when 
lowering (or lifting) the payload. It is a complete 
change of the control method compared to the most 
commonly used solutions, specifically, throttling 
control with volumetric control. A new control method 
can also reduce the oil temperature raising effect 
existing in drives with throttle control. The drive was 
installed on a real scissor lift.
This paper aims to introduce the experimentally 
confirmed system that achieves the assumed speed 
mapping of the plunger cylinder regardless of the load 
during the lowering cycle (active load) of the variable-
ratio mechanism. The proposed drive can be used 
either with high or low nominal power installed. The 
case of active load (e.g., during the lowering phase) 
in hydraulic systems is difficult to control using the 
throttle control method (speed depends on the load), 
and the systems with variable displacement pumps or 
proportional valves used are expensive.
The paper is organized as follows: Chapter 1 
presents the problem statement. Chapter 2 describes 
the presented solution. The test stand and its main 
parameters are presented in Chapter 3. The results 
of the experimental tests are shown in Chapter 4. 
Conclusions are drawn in Chapter 5.
1  PROBLEM STATEMENT
There are many examples of drives when the force 
on the hydraulic cylinder changes during machine 
operation due to the kinematic connections of 
subsequent structural members of the machine. As an 
example of such a stationary machine, a scissor lift 
was considered (Fig. 3). It is a mechanical handling 
device designed for lifting people or payloads to 
a defined height. Different designs of the driving 
system of scissor lifts are presented in [28]. A complex 
geometry results in a variable ratio i, which causes 
a change in a platform speed at a constant speed of 
the hydraulic cylinder. The variable ratio also entails 
a variable force on the drive unit as a function of the 
lifting height (or cylinder stroke), resulting from the 
weight of the structure and the payload being carried. 
The problem of the optimal design of the construction 
and reducing the force acting on the hydraulic 
cylinders is presented in [29] and [30]. The design of 
a scissor lift aims at obtaining the smallest mechanism 
ratio changes, meaning small changes of the working 
platform speed concerning the speed of the hydraulic 
cylinder.
Fig. 3.  Scissor lift
Fig. 3 presents a schematic drawing of the 
discussed scissor lift. The height of the platform 
x
p
 is determined from the length of the hydraulic 
cylinder x
s
. The nonlinear dependence between 
the displacement of the hydraulic cylinder and the 
platform indicates the variable mechanism ratio i, 
calculated according to Eq. (3), where v
p
 and v
s
 are the 
platform and hydraulic cylinder speeds, respectively.
 i
v
v
dx
dx
p
s
p
s
= = . (3)
Furthermore, the lower the platform position, 
the higher pressure in the system. In commonly used 

Strojniški vestnik - Journal of Mechanical Engineering 67(2021)11, 599-610
603 Electro-Hydraulic Drive of the Variable Ratio Lifting Device under Active Load
systems, the platform speed increases inversely with 
the height of the platform. It is related to the use 
of throttle valves to control the lowering payload. 
It should be emphasized that during lowering the 
payload, the electric motor may be off or if it operates, 
the pump may force oil to the tank, for example, 
according to the right position of the directional valve 
5 shown in Fig. 1. However, the dependence of the 
speed on the load in the classical system has an impact 
on the energy consumption. The lowering phase time 
increases with the decreasing load, and thus the supply 
time of the directional valve coil increases.
The pump flow rate Q
p
, according to the 
Eq. (4), depends on parameters of the pump, such as 
volumetric efficiency η
vp
 and displacement q
p
 and 
angular speed of the pump shaft ω
m
.
 Q
q
p
pm
vp



. (4)
The consequence is maintaining the hydraulic 
cylinder speed v
s
 proportional to the oil flow rate Q
p
, 
and inversely proportional to the cross-sectional area 
of the piston A
s
, according to Eq. (5).
 v
Q
A
s
p
s
= . (5)
The use of a drive with a motor fed by FI enables 
the full control of the speed of lowering the platform, 
regardless of the load. To obtain the assumed speed 
of the electric motor, FI is used. Nowadays, most FIs 
are equipped with a vector control mode (VFD), also 
called field-oriented control (FOC), which keeps a 
stable motor speed regardless of its load. FOC mode 
ensures work safety and the ability to work at low 
speeds, for example, during precise load positioning. 
Moreover, this mode allows stopping a hanging load 
at any height without using an electro-mechanical 
motor brake and resuming the movement at any time. 
The correctness of speed mapping is very high (up to 
0.01 %). Therefore, the presented studies are carried 
out using an electric motor fed by the FI operating in 
FOC mode. 
2  SOLUTION CONCEPT
The main features of the new control system, in 
addition to changing the control method from throttling 
to volumetric, are the simplicity and the possibility of 
using it in a typical hydraulic system, equipped with 
a fixed displacement pump, an asynchronous electric 
motor, and hydraulic valves. The elements of the new 
system (Fig. 4) are as follows:
• electric motor fed by FI operating in FOC mode;
• reversible gear pump;
• solenoid operated check valve 2/2 (SOCV);
• open-loop controller (OLC);
• the hydraulic cylinder position sensor (in the case 
of variable-ratio mechanism).
The use of the SOCV allows both lifting and 
lowering of the hydraulic cylinder. During a power 
failure, the valve is closed and prevents the cylinder 
from falling. Lowering is carried out by sending a 
piloted signal to the coil of this valve simultaneously 
with the FI input signal, proportional to the assumed 
speed. The oil flows from the hydraulic cylinder 
through the valve to the hydraulic reversible pump, 
which operates as a hydraulic motor. The electric 
motor is driven by the pump and acts as a generator. 
The assumed speed of the electric motor (and pump) 
is maintained using the FI. The appropriate torque on 
the pump shaft is the result of the FI control system 
operation.
The controller allows declaring any voltage 
function that will be sent to the FI. This can be useful 
during automation, as well as to reduce overloads 
and ensures the safety of people or goods, which can 
increase the lifetime of the system.
Fig. 4.  The hydraulic and control system
The use of a position sensor located on the 
hydraulic cylinder is necessary only when the system 
drives the mechanism with a variable ratio, and the 
working element is controlled (in the presented case: 
the platform). Based on the position sensor mounted 
on the hydraulic cylinder and the known, determined 
because of geometric data, variable ratio, it is possible 

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to control the speed of any selected element of the 
structure (e.g. scissor lift platform). The principle of 
operation of the control system is presented in Fig. 
5. In the OLC algorithm, the current position of the 
hydraulic cylinder x
s
 allows determining momentary 
mechanism ratio i, which is calculated as a polynomial 
function, presented in Eq. (6). 
 ia x
i
n
is
i



0
. (6)
The displacement of the working element 
with assumed speed v
Aset
 is realized by forcing the 
appropriate speed ω
mset
 of the electric motor according 
to Eq. (7).
 
mset Aset
s
p
v
A
q
 . (7)
The reference signal from the OLC to the FI U
ref
 
is proportional to the motor speed and is defined by 
Eq. (8), where k
ω
 is the conversion coefficient, and 
i
min
 is the minimum value of the mechanism ratio:
 U
i
ik
ref
mset
min
 


. (8)
In the case of lowering the payload, the controller 
simultaneously sends binary signals to the solenoid-
operated check valve 2/2 (opening the flow from 
the cylinder to pump) and FI (reverse direction of 
the motor rotation). The oil begins to flow from 
the hydraulic cylinder to a pump that works like 
a hydraulic motor.
Fig. 5.  Open-loop controller (OLC)
3  TEST STAND
Experimental tests were carried out on a custom-
designed scissor lift prototype (Fig. 6). 
The stand was equipped with the drive components 
and the proposed speed control system shown in Table 
1. For the determination of the mechanism ratio, 
a position sensor of the hydraulic cylinder piston rod 
is installed. Additionally, a platform position sensor is 
used to verify the movement of the actuator element 
and the power network analyser is used to measure 
the power consumption of the drive system. The OLC 
has been designed using the LabVIEW software 
installed on a PC co-operating with a multifunction 
input/output (I/O) device. 
Fig. 6.  Scissor lift test stand
The principle of operation of the controller 
is based on the equations described above. The 
mechanism ratio i as a function of the hydraulic 
cylinder displacement x
s
, shown in Fig. 7, was 
described by a 6
th
-degree polynomial. The mechanism 
ratio changes from 13 in the lower platform position 
to 6.6 in its upper position.
Table 1.  Test stand setup
No. Component Parameters
1 PC + LabVIEW Dell Inspiron 17 G3 3779
2 Software NI USB 6434 16 AI, 2 AO, 24 DIO USB
3
Frequency Inverter SX2400-
0R7G-2
0.75 kW /  
0 V to 10 V
4 Motor Simotics GP 1AV1082B 0.55 kW / 1385 rpm
5 Pump XV-0R/0.98 0.92 cm
3
/rev
6
Solenoid-operated check valve 2/2 
EP-08W-05-M-04
350 bar /  
30 l/min
7
Hydraulic cylinder position sensor 
EMAX-000-01.5-2-CAO
1.5 m /  
125 kBit/s
8
Platform position sensor WDS-
3000-P96-CR-TTL
3000 mm /  
11.53 pulses/mm
The presented mechanism is the only case 
that requires an additional sensor. This is due to the 
variable ratio of the mechanism and the necessity of 
determining the ratio to maintain the assumed speed 
of the platform. This entails the need to know the 

Strojniški vestnik - Journal of Mechanical Engineering 67(2021)11, 599-610
605 Electro-Hydraulic Drive of the Variable Ratio Lifting Device under Active Load
geometry of the mechanism and its ratio. Constant 
ratio mechanisms do not require a sensor of cylinder 
displacement.
Fig. 7.  Mec hanism	ratio 	i as a function of the hydraulic cylinder 
displacement x
s
4  EXPERIMENTAL INVESTIGATION AND RESULTS
The experimental tests were divided into two sections:
1.  Comparison of the assumed speeds with measured 
on the stand.
2.  Testing the influence of the load on maintaining 
the assumed speeds.
Fig. 8.  Co m pariso n	o f	a)	sine	and	trapezo idal	functio ns	 
for pressure in the hydraulic cylinder, and b) total hydraulic cylinder 
length during the lowering cycle
Each section was tested separately for the 
assumed speed of the hydraulic cylinder and the 
actuator - a scissor lift platform.
4.1  Verification of the OLC
The scissor lift loaded with a nominal load of 96 kg 
(100 % load) was used for the tests. The platform 
was lowered using two control functions: sine and 
trapezoidal. These functions are the base for further 
studies related to the minimization of dynamic loads, 
in which both types of inputs will be used. Fig. 8a 
presents the pressure change p
s
 in hydraulic cylinders 
during the lowering. It is a visible change of the 
pressure from 4.5 MPa at the start (upper) position to 
almost 7 MPa (sine) and 8 MPa (trapezoidal) at the 
end cycle when the platform is at the bottom position. 
Fig. 8b presents the actual length of the hydraulic 
cylinder x
s
.
a)
 
b)
Fig. 9.  Comparison of the set and measured speeds of the 
hydraulic cylinder (v
s
);	a)	sine,	and	b)	trapezo idal
The waveforms presented in Fig. 9 show the 
speed of the hydraulic cylinder with a) sine, and 
b) trapezoidal control functions. In both cases, the 
difference between the speeds increases with time. 
This is due to the diminishing volumetric efficiency 
of the pump due to the increasing leaks under pressure 

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606 K o suc ki,	A .	–	S ta wiński,	Ł.	–	Mo ra wiec,	A .	–	Go szczak ,	J.
•	 U
ref
	 as a function proportional to the 
assumed speed of the hydraulic cylinder  
v
s
 = –9.15 mm/s with smooth acceleration and 
deceleration 1-second ramp (Fig. 11).
•	 U
ref
 as a function proportional to the 
assumed speed of the platform v
p
 = 60 mm/s 
with smooth acceleration and deceleration  
1-second ramp (Fig. 12).
To facilitate the reading of Figs. 11 to 13, the 
magnifications of the selected areas of the charts are 
placed.
Fig. 11.  Comparison of the set and measured speeds v
s
 of the 
hydraulic cylinder with different loads
The applied ramps reduced the overloads and 
vibrations emerging in the system. This increases the 
service life of the entire machine and affects the safety 
of the transported cargo or the comfort of people.
Both the speed of the hydraulic cylinder and 
the platform maintain the assumed speeds regardless 
of the load. The maximum difference between 
the set platform speed is 10 % and the average is 
6 %. Detailed differences are presented in Table 2, 
where set is reference value of the platform speed, 
maximal actual value of the platform speed, and mean 
arithmetical mean of the actual platform speed during 
steady motion.
growth in the hydraulic system during lowering. The 
maximum difference between the set speed and the 
actual speed is 2.4 mm/s.
The OLC allows continuous adjustment of the 
speed of the hydraulic cylinder and thus the scissor 
lift platform. The assumed and measured waveforms 
of the platform speed are presented, respectively, in 
Fig. 10. As with the hydraulic cylinder, the platform 
speed increases when the platform is lowered. The 
proposed control system does not consider the change 
of efficiency of the pump. This is a field for further 
research and development of the possibilities of the 
presented method of speed regulation. However, in 
this case, the maximum difference between the set 
speed and the actual speed is 5.9 mm/s. 
a)
b)
Fig. 10.  Comparison of the set and measured speeds of the 
scissor lift platform (v
p
);	a)	sine,	and	b)	trapezo idal
4.2  Load Impact on the OLC
The objective of the second section of the experiments 
is to validate the proposed control structure for 
different test cases covering the operating range of 
the drive. Operation of the system is evaluated with 
an external load at the level of (0, 50, 100) % of the 
nominal load (respectively, 0 kg, 48 kg and 92 kg) 
for lowering the platform from a height of 2150 mm. 
The voltage reference functions are carried out in the 
following cases:

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607 Electro-Hydraulic Drive of the Variable Ratio Lifting Device under Active Load
Fig. 12.  Comparison of the set and measured speeds v
p
 of the 
scissor lift platform with different loads
Fig. 13.  Platform displacement x
p
 with different loads
The difference in the position of the platform after 
lowering it over the same period is 58 mm between 
a fully loaded and empty lift. The displacements of 
lowering the platform for different loads are shown in 
Fig. 13.
These values result from a decline in the 
volumetric efficiency of the pump with increasing 
pressure. To eliminate these differences, a pump 
leakage compensation system or close loop control 
system should be used. During the tests, the electrical 
parameters in the line supplying of the stand were 
also recorded. Measurement of instantaneous power 
allowed to determine the energy consumed in each 
cycle. A conventional drive system and a new one 
were used for comparison. 
Table 2.  Comparison of platform speed v
p
 for different loads
Load 
[%]
Platform speed
set v
p
 [mm/s] max v
p
 [mm/s] mean v
p
 [mm/s]
0 –60 –64.33 –62.05
50 –60 –65.73 –62.93
100 –60 –66.10 –63.70
Fig. 14 presents the comparison of power 
consumption during the lowering cycle. To illustrate 
the cycle, the platform displacement is presented 
as well. The actual power consumption of the 
conventional drive is at the constant level during the 
whole cycle, what is visible in Fig. 14a. It results 
from the constant power demand of the device control 
system and the supply of the throttle valve coil. One 
of the disadvantages of this system is the dependence 
of the load on the lowering time, which can reach over 
120 s for an empty platform. The new setup of the 
valve can change these cycle times. Fig. 14b shows 
that the power consumption depends on the platform 
displacement, but all cycles are about 35 s. The 
duration of the cycle depends only on the settings of 
its parameters, such as the assumed speed or the times 
of the individual cycle phases. The FI installed on the 
stand cannot return energy into the power network. 
To estimate the energy balance of a drive capable of 
returning energy to the power network, based on the 
measured hydraulic quantities associated with the 
pump, the power recoverable P
m
 during lowering was 
determined according to dependency:
 Pq p
mp mp mp
   , (9)
where q
p
	 is  pump displacement, ω
m
 actual angular 
speed of the pump shaft, p
p
	 actual pressure on the 
pump outlet, η
p
 pump efficiency, and η
m	
electric motor 
efficiency.

Strojniški vestnik - Journal of Mechanical Engineering 67(2021)11, 599-610
608 K o suc ki,	A .	–	S ta wiński,	Ł.	–	Mo ra wiec,	A .	–	Go szczak ,	J.
To estimate the energy consumption E, the power 
function was integrated according to the Eq. (10):
 EP Pd t
t
m
 

0
. (10)
Fig. 15 presents the comparison of the lowering 
energy of the three systems: 
• conventional with directional and throttle valves,
• new with FI,
• new with FI and energy recuperation estimation.
Fig. 15.  Lowering energy comparison with different platform loads 
and drive systems
It is visible that the ability to free mapping of the 
speed using the new system costs a certain amount of 
energy. Differences reached even about 60 % for the 
system without energy recuperation. It should be noted 
that this is a low-capacity drive and the difference in 
energy demand decreases with increasing load.
When this feature is included, the system gives 
lower energy consumption for the empty platform 
and small loads (from 20 % to 87 %) and energy 
recuperation in case of maximum load. The reduction 
in power consumption results from the fact that 
the motor working during lowering as a generator, 
transfers electric power to the DC link of FI, which in 
common FI’s is lost to the resistor. The FI’s equipped 
with the regenerative unit allows energy to return to 
the grid when the DC link voltage level is too high. 
It causes a real decrease in energy demand, e.g., in 
lowering cycles.
5  CONCLUSIONS
The proposed system of controlling the lowering of 
the hydraulic cylinder maintains the set speed with 
high accuracy. The used method of control simplifies 
the structure of the hydraulic system, among others the 
throttle valve and directional valve are removed. New 
drive allows freely shaping of the speed of the piston 
rod or working element (e.g. platform) depending on 
the needs. However, it causes the increased energy 
         
         
Fig. 14.  Power consumption P and platform displacement x
p
 during lowering cycle: a), c) conventional, b), d) new

Strojniški vestnik - Journal of Mechanical Engineering 67(2021)11, 599-610
609 Electro-Hydraulic Drive of the Variable Ratio Lifting Device under Active Load
consumption in relation to the classical system. The 
presented system can be, although energy efficient by 
using FI equipped with a regenerative unit, despite that 
the scissor lift is a relatively low-capacity machine.
This system has significant advantages over other 
methods of speed control and positioning for lowering 
payload. These are, among others: simple structure, 
low price, the possibility of using any size, and type 
of pump. This means that the presented solution of the 
volumetric control of hydrostatic drives can be used 
without any limitation of a pump size in contrary to 
variable displacement pumps, which are produced in 
sizes above 8 cm
3
/rev. This extends the possibility of 
volumetric control also to low power driving systems. 
Additionally, the possibility of continuous 
adjustment of the pump motor speed allows 
minimizing adverse phenomena occurring during 
acceleration, such as vibrations or overloads.
The presented solution significantly extends the 
possibilities of controlling hydraulic systems in both 
passive and active loads. 
7  NOMENCLATURES
A
D
 cross-sectional area of valve orifice, [m
2
]
A
s
 cross-sectional area of piston, [m
2
]
c
D
  coefficient of flow losses, [-]
ds diameter of hydraulic cylinder, [m]
E energy consumption, [J]
i mechanism ratio, [-]
k
ω
 conversion coefficient, [V·s/rad]
P
m
 recoverable power, [W]
p
p
 actual pressure on pump outlet, [MPa]
Δp
s
 pressure drop between valve inlet and outlet, 
[MPa]
q
p
 pump displacement, [m
3
/rev]
Q
D
 throttling valve flow rate, [m
3
/s],
Q
p
 pump flow rate, [m
3
/s],
U
ref
 reference signal from OLC, [V]
v
Aset
 assumed speed of working element, [m/s]
v
p
 and v
s
 are the platform speed, [m/s]
v
s
 hydraulic cylinder speed, [m/s]
x
p
 height of platform, [m]
x
s
 length of hydraulic cylinder, [m]
η
m
 electric motor efficiency, [-]
η
p
 pump efficiency, [-]
η
vp
 pump volumetric efficiency, [-]
ρ density of hydraulic liquid, [kg/m
3
]
ω
m
 actual angular speed of pump shaft, [rad/s]
ω
mset
 assumed speed of electric motor, [rad/s].
6  REFERENCES
[1] Liu, Y.; Li, W., Li, D. (2018). Review on inlet/outlet oil 
coordinated control for electro-hydraulic power mechanism 
under sustained negative load. Applied Sciences, vol. 8, no. 6: 
art. ID 886, DOI:10.3390/app8060886.
[2] Bednarski, S. (2012). Controlled movement of the hydraulic 
cylinder with active load. Hydraulika I Pneumatyka, no. 2, p. 
10-14. (in Polish)
[3]  Stawiński, Ł. (2011) Hydrostatic systems for driving cylinders 
with variable direction loading of the piston rod. Hydraulika i 
Pneumatyka, no. 1, 17-20. (in Polish)
[4]  Dabholkar, R., Indulkar, S. (2012). Overcenter valves are 
key to hydraulic control. Design World, from https://www.
designworldonline.com/overcenter-valves-are-key-to-
hydraulic-control/, accessed on 2020-11-23.
[5] Hitchcox, A. (2009). The Truth about Problem Valves, 
Hydraulics & Pneumatics. The Penton Media Buildings, 
Cleveland.
[6] Johnson, J.L., (2009). Counterbalance Valve Circuits, 
Hydraulics & Pneumatics, The Penton Media Building, 
Cleveland.
[7]  Zähe, B. (2010). Für einen besseren Wirkungsgrad. Auswahl 
und Verschaltung von Senkbremsventilen. SUN Hydraulik 
GmbH in Erkelenz, Mainz.
[8]  Nordhammer, P.A., Bak, M.K., Hansen, M.R. (2012). A method 
for reliable motion control of pressure compensated hydraulic 
actuation with counterbalance valve. Proceedings of the 
12
th
 International Conference on Control. Automation and 
Systems, p. 759-763.
[9] Nordhammer, P.A., Bak, M.K., Hansen, M.R. (2012). Controlling 
the slewing motion of hydraulically actuated crane using 
sequential activation of counterbalance valve. Proceedings of 
the 12
th
 International Conference on Control. Automation and 
Systems, p. 773-778.
[10]  Dasgupta, K., Watton, J. (2018). Modeling and dynamics of a 
two-stage pressure rate controllable relief valve: A bondgraph 
approach. Int ernatio nal	 Journal	 o f	 Mo delling	 and	 Simulatio n, 
vol. 28, no. 1, p. 11-19, DOI:10.1080/02286203.2008.11442
444.
[11] Stawiński, Ł. (2016). Experimental and modeling studies of 
hydrostatic systems with the counterbalance valves which are 
used in hydraulic lifting systems with passive and active load. 
Eksplo atacja	 i	 Nieza w o dno sc	 -	 Maint enance	 and	 R eliability, 
vol. 18, no. 3, p. 406-412, DOI:10.17531/ein.2016.3.12.
[12] Zhao, L., Xinhui, L., Tongjian, W. (2010). Influence of 
counterbalance valve parameters on stability of the crane 
lifting system. Proceedings of the International Conference on 
Mec hatr o nics	 and	 A ut o matio n, p. 1010-1014, DOI:10.1109/
icma.2010.5589356.
[13] Agostini, T., De Negri, V., Minav, T., Pietola, M. (2020). Effect 
of energy recovery on efficiency in electro-hydrostatic closed 
system for differential actuator. Actuators, vol. 12, Iss. 9, art. 
ID 12, DOI:10.3390/act9010012.
[14] Cochran, K. (2012). Cartridge valve and manifold technologies 
- a component approach to improved energy efficiency. 
Proceedings of the Energy Efficient Hydraulics and 
Pneumatics Conference, Rosemont.

Strojniški vestnik - Journal of Mechanical Engineering 67(2021)11, 599-610
610 K o suc ki,	A .	–	S ta wiński,	Ł.	–	Mo ra wiec,	A .	–	Go szczak ,	J.
[15]  Andersen, B.R. (2009). Energy efficient load holding 
valve. Proceedings of the 11
th
 Scandinavian International 
Conference on Fluid Power, Linkoping.
[16]  Bao, Z. (2019). Study on simulation of system dynamic 
characteristics of hydraulic scissor lift based on load-
sensing control technology. IOP	 Co nf erence	 Series:	 Mat erials	
Science and Engineering, vol. 612, no. 4, art. ID 042036, 
DOI:10.1088/1757-899X/612/4/042036.
[17] Heikkila, M., Linjama, M. (2013). Displacement control of 
a mobile crane using a digital hydraulic power management 
system. Mechatronics, vol. 23, no. 4, p. 452-461, 
DOI:10.1016/j.mechatronics.2013.03.009.
[18] Lyu, L., Chen, Z., Yao, B. (2018). High precision and high 
efficiency control of pump and valves combined hydraulic 
system. Proceedings of the 15
th
 International Workshop on 
A dv anced 	 Mo tio n	 Co ntr o l	 (AMC), p. 391-396, DOI:10.1109/
AMC.2019.8371124.
[19] Minav, T., Immonen, P., Laurila, L., Vtorov, V., Pyrhonen, J., 
Niemela, M. (2011). Electric energy recovery system for a 
hydraulic forklift - theoretical and experimental evaluation. 
IET Electric Power Application, vol. 5, no. 4, p. 377-385, 
DOI:10.1049/iet-epa.2009.0302.
[20] Lovrec, D., Tic, V., Tasner, T. (2017). Dynamic behaviour of 
different hydraulic drive concepts - comparison and limits. 
Int ernatio nal	 Journal	 o f	 Simulatio n	 Mo delling, vol. 16, no. 3, p. 
448-457, DOI:10.2507/ijsimm16(3)7.389.
[21] Avram, M., Spânu, A., Sârbu, V. (2018). Method for controlling 
the hydraulic pump flow following an imposed frequency 
law for AC motors. IOP	 Co nf erence	 Series:	 Mat erials	
Science and Engineering, vol. 444, no. 4, art. ID 042009, 
DOI:10.1088/1757-899X/444/4/042009.
[22] Kosucki, A., Stawiński, Ł. (2016). Studies on hydrostatic 
systems powered by frequency inverters. Proceedings of the 
International Scientific and Technical Conference: Hydraulic 
and Pneumatic Drives and Control, vol. 1, p. 5-16. (in Polish)
[23] Yang, X., Gong, G., Yang, H., Jia, L., Zhou, J. (2017). An 
investigation in performance of a variable-speed-displacement 
pump-controlled motor system. IEEE/ASME	 T ransactio ns	
o n	 Mec hatr o nics, vol. 22, no. 2, p. 647-656, DOI:10.1109/
tmech.2016.2544440.
[24] Padovani, D., Ketelsen, S., Hagen, D., Schmidt, L. (2019). A 
self-contained electro-hydraulic cylinder with passive load-
holding capability. Energies, vol. 292, no. 12, art. ID 292, 
DOI:10.3390/en12020292.
[25] Ritelli, G.F., Vacca, A. (2013). Energetic and dynamic 
impact of counterbalance valve in fluid power machines. 
Energy	 Co n v er sio n	 and	 Management, vol. 76, p. 701-711, 
DOI:10.1016/j.enconman.2013.08.021.
[26] Stawiński, Ł., Kosucki, A., Morawiec, A., Sikora, M. (2019). A 
new approach for control the velocity of the hydrostatic system 
for scissor lift with fixed displacement pump. Archives of Civil 
and	 Mec hanical	 Engineering, vol. 19, no. 4, p. 1104-1115, 
DOI:10.1016/j.acme.2019.06.001.
[27] Stawinski, Ł., Zaczynski, J., Morawiec, A., Skowronska, J., 
Kosucki, A. (2021). Energy consumption structure and its 
improvement of low-lifting capacity scissor lift. Energies, vol. 
14, no. 5, art. ID 1366, DOI:10.3390/en14051366.
[28] Stancek, J., Bulej, V. (2015). Design of driving system for 
scissor lifting mechanism. A cademic	 Journal	 o f	 Manufacturing	
Engineering, vol. 13, no. 4, p. 38-43.
[29] Hongyu, T., Ziyi, Z. (2011). Design and simulation based 
on Pro/E for a hydraulic lift platform in scissors type. 
Procedia Engineering, vol, 16, p. 772-781, DOI:10.1016/j.
proeng.2011.08.1153.
[30] Liu, T., Jian, S. (2009). Simulative calculation and optimal 
design of scissor lifting mechanism. Proceedings of the 
Chinese Control and Decision Conference, p. 2079-2082.