Using Ultracapacitors as Energy-storing Devices on a Mobile Robot
Platform Power System for Ultra-fast Charging
Carlos Arantes
1
, João Sepúlveda
2
, João Sena Esteves
2
, Hugo Costa
1
and Filomena Soares
2
1
Department of Industrial Electronics, University of Minho, Campus of Azurém, 4800-058 Guimarães, Portugal
2
Centre Algoritmi, Department of Industrial Electronics, University of Minho, Campus of Azurém, 4800-058 Guimarães,
Portugal
Keywords: Ultracapacitors, Batteries, Fast Electric Charger, Mobile Robot.
Abstract: The large charging times required by conventional batteries constitute an important limitation in many
applications. The use of ultracapacitors as energy storage elements allows substantially faster charging. This
paper presents a power supply system developed in order to validate the possibility of providing a mobile
robot platform with an electrical energy storage system based on ultracapacitors and batteries, ensuring both
the autonomy and the charging time required by this vehicle. Both simulations results and experimental
results also presented in this paper – validate this possibility. Using exclusively one ultracapacitors
module as energy-storing device of the new power supply system, the mobile platform achieved an
autonomy of 22 minutes after a charging time of 1 minute and 57 seconds. The charging time is less than
10% of the autonomy time. The system also proved its ability to properly charge lead-acid batteries or
nickel–metal hydride batteries, which may be used as energy-storing devices, allowing the mobile platform
to achieve greater autonomy than the one obtained with ultracapacitors (at the cost of larger charging times).
1 INTRODUCTION
Conventional batteries usually require charging
times of, at least, some tens of minutes due to the
allowable values of their charging currents. These
charging times are an important limitation in many
applications, such as electric vehicles. Although
many authors proposed battery fast charging
techniques for different battery types (Petchjatuporn,
2005; Li, 2009; Hua, 2010), charging times cannot
be reduced much further, because the temperature
rises too much, deteriorating the batteries.
The charging currents of ultracapacitors may be
substantially higher than those allowed by common
batteries. So, ultracapacitors charging periods may
be much smaller than those required by batteries.
This paper addresses the possibility of using
ultracapacitors as energy-storing devices when very
reduced charging times are required.
Since batteries have much higher energy
densities than ultracapacitors, they are more suitable
to store large amounts of energy in order to provide
a large autonomy to onboard powered devices.
Regarding this aspect, the difference between
batteries and ultracapacitors may be substantially
reduced in a near future due to the introduction of
new materials (Bernholc, 2010).
The discharging currents of ultracapacitors may
also be substantially higher than those allowed by
common batteries. Due to this fact, ultracapacitors
are able to provide power peaks of significant value.
Several applications integrating batteries and
ultracapacitors on the same system are currently
available (Schneuwly, 2000; Wenzhong, 2005;
Miller, 2007; Awerbuch, 2008; Xiaofei, 2009;
Awerbuch, 2010; Dai, 2010; Haihua, 2010; Lei,
2010; Niemoeller, 2010; JennHwa, 2011; Monteiro,
2011; Musat, 2012). This integration results from the
need of having a single system capable of,
simultaneously:
1. Storing large amounts of energy in the batteries
for the sake of autonomy;
2. Providing power peaks of short duration but
significant value using the ultracapacitors,
extending the battery lifetime (Musat, 2012)
and/or ensuring the proper functioning of the
system (Schneuwly, 2000; Monteiro, 2011).
This paper also addresses the possibility of
integrating batteries and ultracapacitors in the same
device. However, the main reason for doing so is
different from the previously described. Having
Arantes C., Sepúlveda J., Sena Esteves J., Costa H. and Soares F..
Using Ultracapacitors as Energy-storing Devices on a Mobile Robot Platform Power System for Ultra-fast Charging.
DOI: 10.5220/0005061801560164
In Proceedings of the 11th International Conference on Informatics in Control, Automation and Robotics (ICINCO-2014), pages 156-164
ISBN: 978-989-758-040-6
Copyright
c
2014 SCITEPRESS (Science and Technology Publications, Lda.)
batteries released from the requirement of providing
high power peaks remains an important property of a
power supply system. But the main goal of the
integration suggested in this paper is enabling a
vehicle to achieve a reasonable autonomy after a
very reduced charging time, when its energy-storing
devices (batteries and ultracapacitors) become
discharged and there is not enough time to properly
charge the batteries.
The main objective of the power supply system
presented in this paper is to validate the possibility
of providing a mobile robot platform with an
electrical energy storage system based on
ultracapacitors and batteries, ensuring both the
autonomy and the charging speed required by this
vehicle. Starting with fully discharged energy-
storing devices, the mobile platform must achieve, at
least
1. 15 minutes of autonomy after a charging
period not exceeding 2 minutes, when there
is not more time available for charging;
2. Some hours of autonomy, when there are
no charging time restrictions.
Conventional batteries easily accomplish the
second task. For example, the expected autonomy of
the mobile platform powered by a fully charged
12V, 12Ah, 144Wh lead-acid battery ranges from 7
to 9 hours.
So, the main effort was put on studying the
possibility of accomplishing the first task using only
ultracapacitors. That is the reason why the majority
of the results presented in this paper are related to
the use of one 16V, 116F, 4.12Wh ultracapacitors
module. Since batteries are not used in the first task,
this paper may be put in the context of
“environmental-friendly” approaches.
Conventional batteries are, still, much less
expensive than ultracapacitors. However,
ultracapacitors cost has been significantly
decreasing. For example, the cost of the
ultracapacitors module used in this work decreased
34% in a period of about one year. By now, it costs
4.3 times more than the 12V, 12Ah, 144Wh lead-
acid battery also used in this work.
On the other hand, ultracapacitors allow about
1000 times more charging/discharging cycles than
batteries. The rated number of cycles between rated
voltage and half rated voltage (under constant
current at 25ºC) allowed by the previously
mentioned ultracapacitors module is 500000.
The energy-storing device of the new power
supply system, described in Section 2, may be an
ultracapacitors module or a battery. Future versions
of the system will integrate energy-storing devices
of both kinds.
Section 3 presents simulation results performed
before implementing the physical system described
in Section 4. Some experimental results obtained
with the real system are presented in Section 5. The
general conclusions and some considerations
regarding future developments are provided in
Sections 6.
2 SYSTEM ARCHITECTURE
OVERVIEW
The developed power supply system has three
components:
An energy-storing device, which may be an
ultracapacitors module, a lead-acid battery or a
nickel–metal hydride battery;
A charger, intended to transfer energy from the
mains to the energy-storing device;
A voltage regulator, intended to supply a
regulated voltage to the mobile robot platform,
regardless of the voltage of the energy-storing
device.
2.1 The Charger
Figure 1 shows the block diagram of the subsystem
that uses the charger. The energy-storing device may
be an ultracapacitor (or an ultracapacitors module,
like the 16V, 116F, 4.12Wh module used in this
work). The charger is also appropriate for lead-acid
batteries or nickel-metal hydride batteries.
Figure 1: Block diagram of the subsystem that uses the
charger.
In the majority of European countries, the
characteristics of single-phase standard electric
plugs are 230V AC, 50Hz and 16A (3.45kVA). The
charger circuit is fed by a 1.5kVA, 50Hz, 230V to
18V single phase transformer, followed by a 25A
rectifier bridge and a 25mF capacitive filter. A 5W,
10 resistor is connected in series with the filtering
capacitors, to avoid a large start up surge in the
electric current. Two seconds later, the resistor is
bypassed by a timer relay. This results in an
unregulated 24V DC power supply, which feeds a
power electronics converter, with a permitted ripple
of 5V and a rated power of 300W.
The power electronics converter of the charger is
a step-down DC-DC converter working with pulse
width modulation current-controlled voltage source,
at a switching frequency of 9.7kHz. The input is a
non-regulated 24V DC voltage and the output is a
regulated adjustable DC current. The control system
is a PI (proportional + integral) controller with a
sampling frequency of 1kHz, with a proportional
gain (Kp) of 8.2 and an integral gain (Ki) of 10.
Figure 2 shows the step-down model developed with
PSIM simulation software.
In the converter shown in Figure 2, diode D5
ensures that the energy always flows one way,
towards the battery or the ultracapacitors module.
Diode D6, resistor R17 and capacitor C10 form a
RCD snubber whose goal is to mitigate the
overvoltages that occur when the MOSFET is turned
off. Regarding snubber parameters calculation, a
maximum current of 20A and a parasitic system
inductance of 100H were considered. In this
situation, capacitor C10 (150F, 100V) must be
capable of absorbing the energy stored in the
parasite coil through D6 and discharged through
resistor R17 (1Ω, 5W). Capacitor C4 (10F, 100V)
is a polypropylene capacitor that also contributes to
the mitigation of overvoltages.
Figure 2: PSIM model of the step-down converter of the
charger.
L1 is the output filter coil (14.6mH, 170mΩ at 1
kHz) that allows smoothing the current around an
average adjustable value. D7 is a diode that ensures
the path to the current when the MOSFET is turned
off. The diodes used are the schottky MBR1660 and
the MOSFET used is the P80PF55.
2.2 The Voltage Regulator
Figure 3 shows the block diagram of the subsystem
that uses the voltage regulator. The objective of this
component is to supply energy to the mobile robot
platform from the energy previously stored in the
battery or in the ultracapacitors module.
The main characteristics of the load (mobile
robot platform) are a nominal voltage of 12V and a
maximum power of 30W. Since the power supplied
to the mobile platform is mainly used to drive DC
motors, the voltage ripple is not a very relevant
issue.
Unlike a battery voltage, which is reasonably
constant during discharging process, a capacitor
voltage is directly proportional to its charge, causing
a large variation on its voltage when it supplies a
load. So, when the ultracapacitors module is used as
energy-storing device, a voltage regulator is required
in order to keep the load voltage set to the intended
value of 12V independently of the ultracapacitors
module voltage.
The voltage regulator also allows using different
types of batteries, with different rated voltages.
The voltage regulator consists of two DC-DC
converters: a step-up followed by a step-down. This
topology was preferred over the step-up/down
topology because, in that case, the output voltage
would have reversed polarity in relation to the input
voltage, and this would require the use of isolated
voltage sensors for measuring. With the
implemented topology, the measurement is carried
out using a simple resistive voltage divider.
The step-down sub-circuit maintains the output
voltage set on the intended value as long as the input
voltage remains above that value. When the input
voltage becomes lower than the output intended
value, the step-up sub-circuit elevates it to the output
intended value. There are still situations when none
of the sub-circuits operate as DC-DC converters, as
is the case in which the input voltage is between
13V and 12.5V. In this case, the step-up MOSFET
stays off while the step-down MOSFET stays on.
This results in a varying output voltage but, in
practice, the effect is negligible.
The two converters never switch simultaneously,
as this would entail unnecessary additional power
losses. Figure 4 shows the voltage regulator model
developed with PSIM simulation software.
In Figure 4, diode D1 ensures that the energy
only flows from the input to the output. Diode D2
provides a path for the current when the MOSFET
belonging to the step-up sub-circuit is open. L1 is a
coil belonging to the step-up sub-circuit, with an
inductance of 168.7H and a resistance of 170mΩ at
1kHz. L2 is a coil belonging to step-down
sub-circuit and has an inductance of 132.4H and a
resistance of 148mΩ at 1kHz. Capacitors C1 and C2,
both of 1mF, form capacitive filters that are intended
to minimize the ripple caused by high switching
frequencies of both MOSFETs.
The control algorithm consists of a PI controller
with proportional gain of 1 and integral gain of 10
runs the control algorithms of both sub-circuits. The
input voltage, read through one of the channels of
the microcontroller ADC, allows the selection of the
sub-circuit to be switched at any moment. The main
control cycle runs at a frequency of 1kHz, while
semiconductors (MOSFETs) switch at a frequency
of 7.8kHz.
Figure 3: Block diagram of the subsystem that uses the
voltage regulator.
Figure 4: PSIM model of the DC-DC converters of the
voltage regulator.
3 SIMULATION RESULTS
The PSIM simulation software was used to validate
all the developed blocks of the entire system, before
any circuit implementation and execution of any
experimental tests.
The simulations results also made possible
making minor adjustments to the controller gains,
allowing a better performance.
The most important simulations results are
presented in this section.
3.1 The Charger
The charger was simulated when feeding an 116F
ultracapacitor. A reference current of 20A and a
final voltage of 15V were considered. Figure 5
shows that the ultracapacitor charging is performed
successfully.
3.2 The Voltage Regulator
The voltage regulator was simulated considering an
116F ultracapacitor charged up to 15V, supplying a
12V, 2A load. In Figure 6, it is visible that the
circuit draws more current from the ultracapacitor as
its voltage drops, in order to maintain constant the
output power.
In Figure 7, the load current and voltage are
shown, when the voltage regulator is fed by a 116F
ultracapacitor previously charged up to 15V. Load
voltage and current values remain approximately
constant, except on a very short time period around
the 190th second, when slight drops in both voltage
and current are observed. This is due to an input
voltage interval where none of the DC-DC
converters step-up or step-down is switching, in
order to improve energy efficiency. However, these
small drops do not disturb the proper functioning of
the mobile platform.
Figure 5: PSIM simulation results of the charger when
charging an 116F ultracapacitor up to 15V.
Figure 6: PSIM simulation results of an 116F
ultracapacitor voltage and current, when the regulator
feeds a 12V, 2A load.
Figure 7: PSIM simulation results of load voltage and
current when an 116F ultracapacitor feeds the voltage
regulator.
4 SYSTEM IMPLEMENTATION
In this section, a detailed description of the real
system implementation is made. The implementation
was performed after simulating all the system
components in PSIM, and consistently obtaining the
desired results.
4.1 Hardware
The charger module is capable of charging 12V
lead-acid batteries, 9.6V nickel-metal hydride
batteries and 16V ultracapacitors. It is quite heavy
and bulky, because a 1.5kVA, 50Hz transformer was
used. So, it was decided that the charger would stay
out of the mobile platform, working as a charging
station. This strategy is valid, because the mobile
platform is able to move itself to the charging station
when recharging is needed.
The final version of the charger module was
implemented on a printed circuit board (PCB).
Figure 8 shows the charging of a 16V, 116F
ultracapacitors module, at a reference current of
20A.
The voltage regulator module is capable of
supplying a regulated 12V voltage to the mobile
platform using batteries or ultracapacitors as energy
sources. Its input voltage may be between 16V and,
approximately, 1.5V (this lower limit is mainly used
by ultracapacitors).
Figure 9 presents the voltage regulator module
implemented on a PCB. The DC-DC converters
diodes, MOSFETs and coils are clearly visible; the
AVR ATmega 328P microcontroller from Atmel
Corporation and the dedicated crystal are also
noticeable. Some LEDs provide visible signals.
The voltage regulator module and its power
source (batteries or ultracapacitors) are supposed to
be installed on board the mobile platform, which has
been done. Figure 10 shows a previously charged
16V, 116F ultracapacitors module serving as the
energy source of a four-wheeled mobile platform.
Figure 8: Charger module charging a 16V, 116F
ultracapacitors module at a reference current of 20A.
Figure 9: Voltage regulator module.
Figure 10: Voltage regulator module and a 16V, 116F
ultracapacitors module installed on board a four-wheeled
mobile platform. The picture was taken with a long
exposure and it is noticeable that the wheels are turning.
4.2 Software
As mentioned before, the charger module must be
capable of charging ultracapacitors and different
kinds of batteries. And the voltage regulator module
must provide a regulated 12V voltage to the mobile
platform. The PI control algorithms of both modules
were implemented on AVR ATmega 328P
microcontrollers.
The operation mode of the charger module depends
on the type of energy-storing device to be charged.
The following algorithms have been chosen for
implementation because they are the most used for
each type of energy-storing device:
The NDV (Negative Delta Voltage) for
nickel-metal hydride batteries;
Constant current followed by constant
voltage for lead-acid batteries;
Constant current until voltage reaches a
predetermined value for ultracapacitors.
Regardless of the type of energy-storing device,
charging time is not controllable because it depends
on the characteristics of the energy-storing device
and its initial state of charge.
A user-friendly interface with a LCD (Liquid
Crystal Display) and some push buttons was also
developed, allowing the user to select the type of
energy-storing device to be charged and define
charging parameters. Charging options include: type
of battery; battery voltage; battery charging current;
battery capacity; ultracapacitor maximum voltage
and ultracapacitor charging current.
The control algorithm flowchart of the voltage
regulator module is shown in Figure 11. This
module is also able to detect load overcurrents and,
if necessary, automatically shuts down to prevent
damages to the system components.
5 EXPERIMENTAL RESULTS
A few experimental tests were made to the system in
order to ensure the correct functioning of each
module and, also, to further validate the proposed
concept.
5.1 The Charger
The charger module was tested with three energy-
storing devices:
One 16V, 116F, 4.12Wh ultracapacitors module;
One 12V, 12Ah, 144Wh lead-acid battery;
One 9.6V, 800mAh, 7.68Wh nickel-metal
hydride battery.
The expected autonomy of the mobile platform
powered by the tested nickel-metal hydride battery
after a full charge is rather low. It ranges,
approximately, from 30 to 35 minutes. The purpose
of using this battery is just to verify the ability of the
charger to work properly with nickel-metal hydride
batteries.
The results show that the maximum error
resulting from the difference between the desired
value and the actual value of a battery charging
current never exceeded 4%. The end-of-charge
algorithm worked as expected for both types of
batteries. The charging times are, approximately, 8
hours for the lead-acid battery and 4 hours for the
nickel-metal hydride battery.
In ultracapacitors charging mode, the charger
module imposed a current around 20A, charging the
116F ultracapacitors module up to 15V in 1 minute
and 57 seconds. Since the difference between the
reference current and output current never exceeded
700mA, the resulting maximum error is 3.5%.
5.2 The Voltage Regulator
The voltage regulator module, powered by the 116F
ultracapacitors module previously charged up to
15V, was tested on the mobile platform. The tests
were performed on a flat and levelled ground. Under
these operating conditions, the platform absorbed,
approximately, 0.8A while keeping a constant speed
of 62.8cm/s. The system worked at full performance
for 22 minutes. It was also found that, under the
conditions described above, the voltage regulator
succeeded in imposing the required output voltage of
12V until its input voltage, provided by the
ultracapacitors module, dropped to about 1.5V.
Some tests with excessive overcurrents at the
load were also conducted. In all tested situations, the
system was able to detect the overcurrent and did
shut down before the occurrence of any damage.
6 CONCLUSIONS AND FUTURE
DEVELOPMENTS
A power supply system for a mobile robot platform
has been developed. The system may use
ultracapacitors, lead-acid batteries or nickel-metal
hydride batteries as energy-storing devices.
It proved its ability to charge one 116F, 4.12Wh
ultracapacitors module up to 15V in 1 minute and 57
seconds. The system also proved its ability to
properly charge different types of batteries,
following the reference current with an error less
than 4%, and effectively detecting the full charge
state. The algorithms used in this project for battery
charging, sought to diminish charging times, but
without causing significant battery deterioration.
Both simulations results and experimental results
presented in this paper validate the possibility of
providing the mobile robot platform with an
electrical energy storage system integrating
Vi is the measured input voltage
Vo is the measured output regulated voltage
Kp and Ki are the gains of the PI controller
h is the sample period
Figure 11: Control algorithm flowchart of the voltage regulator module.
ultracapacitors and batteries. Such integration,
suggested as future work, would ensure both the
autonomy and the charging speed requirements of
the platform. In fact, using exclusively one 116F,
4.12Wh ultracapacitors module as energy-storing
device, the mobile platform achieved an autonomy
of 22 minutes after a charging time of 1 minute and
57 seconds, exceeding the autonomy required by the
platform when the charging time is limited to 2
minutes. It should be noted that the charging time is
less than 10% of the autonomy time. On the other
hand, the expected autonomy of the mobile platform
powered by a fully charged conventional 12V,
12Ah, 144Wh lead-acid battery, like the one used in
this work, ranges from 7 to 9 hours, fully satisfying
the autonomy required by the platform when there
are no charging time restrictions.
Integrating ultracapacitors and batteries in the
same power supply system also would release the
batteries from the requirement of providing high
power peaks, extending their lifetime.
Battery energy densities are much larger then the
ones offered by ultracapacitors but the introduction
of new materials may reduce the difference in a near
future.
An ultracapacitor is much more expensive than a
battery with the same energy storage capacity.
However, ultracapacitors cost has been significantly
decreasing. Furthermore, because ultracapacitors
allow much more charging/dicharging cycles than
batteries, the cost difference between the two
components is mitigated.
In addition to the suggested integration of
ultracapacitors and batteries in the same power
supply system, the following tasks are suggested as
future work: implementing control algorithms
capable of identifying the battery type to be charged
and its voltage; using a high-frequency transformer
instead of a 50Hz transformer, which would reduce
the cost, size and weight of the charger module,
allowing its installation onboard the mobile
platform; developing more compact PCBs using
surface-mount devices (SMD).
ACKNOWLEDGEMENTS
This work has been supported by FCT Fundação
para a Ciência e Tecnologia within the Project
Scope: Pest-OE/EEI/UI0319/2014.
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