VIBRATION HARVESTING IN RAILWAY TUNNELS
M. Wischke*, G. Biancuzzi, G. Fehrenbach, Y. Abbas, P. Woias
Laboratory for Design of Microsystems, University of Freiburg – IMTEK, Germany
*Presenting Author: wischke@imtek.de
Abstract: We report on the vibration characteristics, and a piezoelectric generator for harvesting electrical energy
to power wireless sensor nodes, inside railway tunnels. Extensive investigations have shown that the railway
sleeper is favorable for placing a vibration harvester concerning vibration levels and mounting space. A shock
resistant double-side suspended piezoelectric harvester has been tested with true train vibrations. In average
260 µWs have been scavenged per train, which is sufficient energy for simple wireless communication. To avoid a
deadlock of the sensor node, we have designed a new ultra low power interface circuit.
Keywords: vibration harvesting, railway tunnels, piezoelectric
INTRODUCTION
The structural integrity of traffic tunnels after
harmful incidents is a decisive factor for all further
measures, e.g. the dispatch of rescue forces. It would
be extremely helpful to retrieve on-site information on
the status of the building via a distributed sensor
system [1]. However, a full-scale monitoring of civil
underground infrastructure requires a large number of
embedded sensors [2]. Equipping a tunnel with a
monitoring sensor grid requires a large effort due to
the hardly accessible location. Thereby, the sensor
nodes might be embedded completely into the host
structure, so that no wired connection is possible.
Scavenging ambient energy to charge or replace builtin batteries [3] opens up the application of autonomous
wireless sensors in such remote locations.
In addition to the alternative power supply an
extremely economic handling of the available energy
is indispensible. Modern ICs and MCUs can operate
with voltages down to 1.8 V. Nevertheless when the
supply voltage drops down to the threshold range,
CMOS devices are draining uncontrolled current.
Thereby, the power consumption can exceed the
generators output and the system becomes deadlocked.
Hence, a faultless start from 0 V is required.
To address the demand on structural health
monitoring (SHM) of traffic tunnels, we have
investigated the traffic induced vibrations as an
alternative power source. For scavenging the electrical
energy, a robust piezoelectric generator is utilized.
Equipped with a novel power interface the presented
power unit can operate from 0 V.
Arlberg-Tunnel
The Arlberg railway tunnel is an old and historic
building. It is situated between Klösterle and St. Anton
am Arlberg in Austria and operated by the Austrian
rail agency (ÖBB).
Lötschbergbasis-Tunnel
The Lötschbergbasis-Tunnel is the youngest long
distance and high speed tunnel in Europe and connects
Frutigen with Raron in Switzerland. The tunnel is
operated by the BLS AG.
Tab. 1: Details of the investigated tunnels.
Length
Completion
Walls
Tracks
Traffic
Speed Limit
Traffic volume
Arlberg
Lötschbergbasis
10.65 km
1884
Natural stones
Ballast, fixed track
2 rails, opposing
100 km/h
70-90 trains/day
34.56 km
2007
Concrete
Fixed track
Partial alternating
200 km/h
80-110 trains/day
Of major interest for micro-generators is the rate
of vertical acceleration induced by passing trains to the
rail track or deck, and the propagation of the vibration
to the tunnel walls. Acceleration sensors from Kistler
(8704B-series and 8636C5) were mounted to the rail,
the sleeper, the base and the tunnel wall.
VIBRATION IN TUNNELS
Tunnels are frequently the bottlenecks in
transportation infrastructure and therefore very busy.
Thus, they are hardly accessible for measurements and
the systematic characterization of numerous tunnels is
almost impossible. The vibrations in two railway
tunnels have been investigated for the purpose as
alternative power source. A brief introduction of the
tunnels is given and Table 1 summarizes their main
features.
Fig. 1: Acceleration sensors mounted at the track in
the Lötschbergbasis-Tunnel.
The AC signals were recorded with a DT9836
module from Data Translation, featuring simultaneous
16-bit resolution at 225 kHz per channel. In the
railroad tunnel, a sensor mounted at the rail was used
as a trigger to start the data recording. Thus, each
passing train was detected individually. The overall
detection period in each tunnel was in range of 20-24 h
and the induced vibrations of approx. 70 trains were
recorded respectively. Figure 1 shows the acceleration
sensors on the railway track in the LötschbergbasisTunnel.
Fig. 2: Acceleration of the railway sleeper in the
Arlberg-Tunnel during the passage of a freight train.
The AC signals and the frequency spectrum from
each sensor were processed with Matlab. The
comparative investigations of all sensors in both
tunnels have shown that only minor vibrations
(< 0.04 m/s2) occur at the tunnel walls. This is due to
the good vibration damping of the railway. Hence,
vibration harvesting at the tunnel wall will provide
insufficient energy for the intended application.
However, the railway sleeper is a promising location
for a micro-generator because of sufficient mounting
space. Figure 2 shows the acceleration of the railway
sleeper in the Arlberg-Tunnel when a freight train
passes. Accelerations larger than 100 m/s2 are induced
by the train. Figure 3 shows the superimposed
frequency spectrum of the vibrations from 67 passed
trains. It can be seen that almost every frequency is
present in the spectrum. Therefore the frequency
specific accelerations are by orders smaller than the
acceleration in the time domain (Figure 2). The train-
induced vibrations at the railway sleeper are subjected
to the stimulation vehicle, i.e. the wagon type, the axle
weight and the quality of the wheels, which causes
variations in frequency spectrum. However, the
superimposition in Figure 3 shows increased
acceleration levels for frequencies around 600 Hz.
VIBRATION HARVESTER
A generator with an inertial mass-spring design is
used to harvest energy from the accelerations up to
0.35 m/s2 around 600 Hz. Concerning maximum
output power, a harvester with a low damping
coefficient would be preferable. But low damping
causes a small bandwidth, which will often lead to a
frequency mismatch. As a compromise a generator
with moderate a Q-factor facilitates a suitable
bandwidth and performance.
The design of the harvester is depicted in Figure 4.
The generative piezoelectric composite is suspended
on both sides with solid-state hinges. This double-side
suspension provides a uniform curvature of the entire
piezoceramics which maximizes the power output.
Furthermore, the design features a high shock
resistance which is important to sustain the high
impact peaks as shown in Figure 2. The seismic mass
is attached to the center of the oscillator and is utilized
to adjust the resonance frequency of the harvester to
the major peak at 600 Hz.
Fig. 4: Schematic of the double-side suspended
piezoelectric generator.
The harvester is fabricated as a piezo-polymercomposite. Two PZT ceramics, the spring steel hinges
and the electrical contacts are integrated in a single
casting step of a two compound epoxy. Cubic magnets
are used as proof mass. The dimensions of the
piezoceramics are 20x5x0.26 mm3 and the outline of
the packaged prototype measures 36x21x17 mm3.
Fig. 3: Superimposed frequency spectrum of 67 trains in the Arlberg-Tunnel.
The harvester was characterized in the lab with
an electromagnetic shaker. Under sinusoidal
excitation with 10 m/s2 up to 300 µW are scavenged
at resonance with optimal load resistance [4]. Figure
5 shows the harvester mounted onto the test rig.
For the majority of the 20 trains, the capacitor
voltage raises above the 2 V, which is a critical level
to operate CMOS ICs. The stored electrical energy in
the capacitor voltage for this test is shown in Figure 8.
In average 260µWs have been scavenged per train.
With a low voltage design this is sufficient energy for
a simple wireless sensor node as demonstrated in [5].
Fig. 5: Packaged harvester on the test rig.
To test the applicability the recorded acceleration
of the railway sleeper from the measurements in the
tunnels were played back on the shaker. A diode
bridge rectifier and a storage capacitor were
connected to the harvester and the capacitor voltage
was monitored. Figure 6 shows the overlay of the
sleeper vibration and the capacitor voltage for a
single train passage. It can be seen, that the voltage
increases stepwise at the high vibration peaks.
Fig. 6: Charging of the storage capacitor from a
single train passage from the Arlberg-Tunnel.
As mentioned before, each train induces slightly
different vibrations to the railway sleeper, which
affects the charging curve. Hence the measurements
were repeated for 20 different trains from the
Arlberg-Tunnel. The results are shown in Figure 7.
Fig. 7: Charging curves of the storage capacitor for
20 different trains from the Arlberg-Tunnel.
Fig. 8: Stored energy in the capacitor for 20 different
trains from the Arlberg-Tunnel.
POWER MANAGEMENT
Utilizing ambient vibrations to power an electrical
system requires a power processing and an
economical handling of the available energy. For
rectification of the AC voltage from the
piezoceramics a trade-off between the threshold
voltage and the leakage current of the diodes has to
be made. A good choice are the Schottky diodes
HSMS-282x [6]. The charge from the generator is
collected in a storage capacitor of 100 µF.
Modern sensor and RF ICs are throughout
fabricated in CMOS technology. Hence they need a
certain supply voltage for proper operation. When the
supply voltage drops below the threshold voltage
uncontrolled states occur and CMOS electronics
drain much higher currents than under normal
operation. If the CMOS devices are connected
directly to the storage capacitor this phenomena can
deadlock the whole sensor node when the subthreshold current exceeds the generator performance.
Therefore the sensor electronic has to stay
disconnected from the storage until the voltage
reaches a suitable level. Due to the energy
consumption of the sensor electronics the capacitor
voltage will decrease. To enable the extraction of
energy from the storage capacitor for powering the
sensor, the load switch has to feature an “off” voltage
lower than the “on” voltage. Further, the supply
voltage for the CMOS electronics should be kept
constant. Figure 9 shows the block diagram of the
used power interface.
Fig. 9: Block diagram of the power interface.
For the energy autonomous operation, it is also
mandatory that the power switch operates properly
from 0 V (empty storage capacitor) and for the reason
of energy conservation the leakage current has to be
very low.
We have designed a power switch circuit that
combines the voltage detection and the switching
function. For the preliminary test the “on”-voltage
was set to 2.4 V and the “off” voltage of 1.9 V
respectively. The power switch consumes less than
100 nA at 2 V. The output voltage is set to 1.8 V by a
voltage regulator (LDO) from Torex [7]. In total, the
power interface consumes less than 3 µW and
guarantees the recommended operation condition for
CMOS sensor and RF circuit.
DISCUSSION
The train-induced vibrations inside railway
tunnels were explored by acceleration measurements.
High accelerations occur at the railway track when a
train passes (see Figure 2), while the tunnel wall
remains almost unaffected. The frequency spectrum
in Figure 3 shows the ambient character of the
vibrations: various frequencies, but with low
acceleration levels. Around 600 Hz higher vibration
levels are found. This peak is specific for the fixed
track Arlberg-Tunnel. The frequency spectrum from
the Lötschbergbasis-Tunnel looks different and also
higher vibrations levels are found at other
frequencies. Each tunnel and track has an individual
frequency spectrum so that the resonance frequency
of a harvester always has to be adjusted by the size of
the seismic mass respectively. Furthermore, each
train shows a slightly different frequency spectrum
and frequencies with maximum acceleration levels.
From the ambient character, the following
challenging demands on the harvester are obtained:
the design must feature a suitable bandwidth to
respond to the vibration of each train and the design
must be robust to sustain the impact accelerations of
more than 100 m/s2. The double-side suspension of
our harvester (Figure 4) ensures the required shock
resistance. The harvester was fabricated in a low cost
technology by manual insert casting. The top cover of
the package (see Figure 5) is a PCB board which will
carry the currently remote power interface circuit in
future. Thus a more compact size will be achieved.
Figure 6 shows that the storage capacitor can be
charged to a suitable voltage level from true train
vibrations. The stepwise charging indicates that only
at the high vibration peaks the harvester is operating
temporarily at resonance. For a more continuous
operation either the bandwidth of the harvester has to
be increased or a resonator array has to be utilized.
However, Figure 7 and Figure 8 proof, that with the
current design enough energy to power a simple
wireless sensor can be scavenged from the train
vibration.
To minimize the losses of the scavenged energy
we have carefully selected the electrical components.
Low leakage, low threshold diodes were chosen for
the bridge rectifier. Even more important is the use of
a power interface that avoids the energy consumption
by malfunction of any sensor node component. By
merging the voltage detection and the load switch in
one circuit, the power consumption was reduced
drastically. With its hysteresis and the capability to
start from 0 V, our power interface fits excellent all
the demands of a link between the harvester and the
sensor node electronics.
CONCLUSION
The presented PEG and the power interface form
a smart unit that can directly supply wireless sensor
nodes from traffic caused vibrations in railway
tunnels. A promising application is high resolution
localization of the train inside the tunnel.
OUTLOOK
Future work is focused on the increase of the
harvester’s bandwidth and the integration of the
sensor and RF circuit into the harvester package and
the application in the railway tunnel.
ACKNOWLEDGEMENTS
The authors would like to thank Mr. Spiss (ÖBB)
and Mr. Hartleitner (ÖBB), and Mr. Stadelmann
(BLS AG) for the access to the tunnels and the
awesome cooperation.
This work is part of AISIS, funded by the BMBF
in the high-tech strategy for protection of
transportation infrastructure.
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