Fibre optic sensors are excellent tools to use for monitoring high-voltage current collectors. Because of their small cross section and electrical neutrality, they are easily integrated into the current collector strip and are well specialized for detection of high-speed load events. The conventional contact force measurement with four force sensors below the collector strips can also be simplified by using fibre optic force and acceleration sensors.
The deregulation of the European Union (EU) railway market and the necessary interoperability of railways and tracks has increased the need for easy-to-use online monitoring of railway tracks and catenaries (in the best case available daily). Great efforts have been put into the development of overhead contact line (OCL) monitoring systems. Permanent monitoring of the load conditions allows the prediction of the resulting wear, and helps to optimize service and repair cycles on the OCL.
Some measurement techniques for OCL monitoring, such as contact force measurements between pantographs and the OCL, are already well established. Boundary conditions are defined in a European standard (EN 50317). Other techniques, for instance the detection of disturbances (misalignments or intruders), are still under development or under optimization.
For heavy load trains or steep tracks, some trains are driven with two locomotives and two pantographs. Because of strong dynamics of the catenary, this can often lead to contact loss of the second pantograph. In such cases, measurement and control of the contact force is of high interest.
Measurements at the interface between OCLs and current collectors with electrical sensors are complex and prone to interference from the high electrical voltages and the strong changes in electric and magnetic fields. In contrast, fibre optic sensors have the potential of easy access to such measurements due to their small size, immunity to electromagnetic fields, and their intrinsic electrical insulation. For this reason, research is currently underway to utilize fibre optic sensors for monitoring purposes in current collectors and OCLs (Schröder et al., 2012; Theune et al., 2004; Boffi et al., 2009).
Maladjusted catenary elements or intruders into the catenary region cause hits on the current collectors. They can damage the carbon of the collector strip and cause serious traffic incidents. In Fig. 1, two used collector strips are shown with their typical stress marks. At the edges, smaller or bigger parts of the carbon have been chipped away. The lower photograph shows a seriously damaged collector strip. Such a strip can seriously damage the OCL.
The aim of this sensing system is the detection of low-level hits already below the damage threshold, in order to identify sources of damages in a very early state and to provide timely maintenance on demand.
Fibre Bragg grating (FBG) optical fibre sensors and especially draw tower FBG (Chojetzki et al., 2005) are the preferred kinds of sensors for monitoring purposes on this interface (Schröder et al., 2013; Comolli et al., 2010). This is due to their ability to have many sensor elements multiplexed along a single optical fibre, and to the fact that the quantitative load result is derived from a spectral shift, which is immune to light intensity variations. Currently, such investigations are focused on the optimization of the sensors and their instrumentation. In this paper, results are reported from the investigation of two different measuring techniques: quantitative logging of (i) fast hit events and (ii) the stationary acceleration-compensated contact force measurement (Fig. 2).
There are different ways known to interrogate load results from FBG sensors. The sensor system used in these studies is based on a broadband light source and a line-based compact spectrometer with a photo-detector line, comprising depolarization components, fibre optic connectors to four sensor lines, fast- and low-noise digital signal processor (DSP) electronics, Ethernet data communication, and a break-free power supply (Fig. 3).
Top: used collector strip with clear abrasion marks on the edges. Bottom: used collector strip with serious damage.
All these components are placed in a box with dimensions of
18 cm
Because of their convenient small dimensions, the integration of fibre optic sensors, and especially of fibre Bragg gratings, is easily achieved inside the current collector. In this case, the sensors are very close to the interacting interface between the collector strip and the overhead contact line. The closer the sensors are to the interface, the more accurately the monitoring of fast hits can be performed, because signal delays and mechanical damping are minimized. An integration of the sensor fibre into the interface between the carbon bar and the aluminium carrier before their assembly showed the best sensing performance and safety for the sensors.
The collector strip with integrated FBG sensors (shown schematically in Fig. 4) has provided outstanding application results for the detection and measurement of hard hits on the carbon bar in the driving (horizontal) direction.
Overhead contact line in contact with a pantograph head containing two collector strips with integrated sensors. Additional force sensors are included below the collector strips.
The FBG sensor interrogating system contains a broadband light source (superluminescent diode), which is depolarized with an appropriate fibre optic depolarizer. The reflected signals from the FBGs in the four sensor heads are spectrally analysed in a compact spectrometer containing a mode scrambler for optimal entrance slit illumination, an imagine diffractive grating, and a line detector (charge-coupled device – CCD – for the “slow” version, CMOS for the interrogation speed of up to 5 kHz). The heart of the interrogator electronics is a digital signal processor (DSP), which controls the intensity of the light source via pulse duration modulation (PDM), corrects the temperature dependence, evaluates the peak spectra using appropriate sub-pixel approximation, integrates the position data, and sends the sensor data via an Ethernet interface to a displaying and saving device.
Experiments showed two principal types of hit events: “soft hits” and “hard hits”.
“Soft hits” can be caused by short-term locomotive accelerations, e.g. from
rail steps or vibration of the pantograph on the catenary. Typical
frequencies are 220 to 250 Hz with corresponding slow peak rise times of
Collector strip with schematic representation of integrated fibre optical sensors (red). They are positioned on the boundary between the aluminium profile and the carbon bar, close to the edge, and were used successfully to detect hits in a horizontal (driving) direction.
Sensor response from a hit with a metal pendulum on the carbon bar (“hard hit”).
Signal detected during a test run. Passing a section insulator at
37 km h
“Hard hits” are characterized by short rise times (50 to 200
The example in Fig. 5 was measured under laboratory conditions: a hit on the
carbon bar was executed with a metal test pendulum. The hard hit on the
carbon excited transversal and longitudinal surface waves, which spread at
velocities of 3.3 and 5.7
These “hard hits” can usually not be detected by sensors far from the
interface, below the collector strips, but only with integrated sensors,
because of the strong signal attenuation. The sensor system
must interrogate at data rates of at least 5000 measurements s
From such laboratory tests, the dependencies between signal amplitude and hit
energy have been found to be linear for these collector strips, with a slope
of 74
For an evaluation of hit measurements, two factors were identified as most
important. The first one is the hit type (“soft” or “hard” hit); the
second one is the hit intensity. Figure 7 shows a part of a track measurement
with an extraordinarily high density of “events” and a variety of
intensities. Red arrows mark the events, which were found by their
characteristics to be hard hits. Hard hits can occur with high and low
intensities. To classify these hits, an injury level had to be defined. It
should be set below but close to the destruction level. A general definition
of a destruction level only by a measured hit energy is not possible, because
other factors, such as hit area (close to the edge, hit with thin tip) and
the history and quality of the carbon material, as well as the construction
of the fixation of the collector strip within the pantograph, play important
roles. In laboratory experiments with new collector strips hit with 50 J at
the same point with a large-area mass, most strips crack after the third hit.
But when a mass with a spherical tip is used, hit energies of 20 J can chip
away parts of the carbon. Therefore the highest alarm level was set at a hit
energy of 20 J. For the collector strips under investigation, this would
correspond to a strain maximum of about 1500
A first concept sought to use the collector strip as a bending beam in order to transform stationary contact forces into strain, which can then be measured. However, an issue arose due to the low thermal conductivity of the upper strip material (carbon) and the different thermal elongation compared to the aluminium carrier sheet. This led to local heating by sunlight or a high electrical current, causing deformation of the strip, which was detected much earlier by the strain sensors compared to the true temperature change. The resulting errors are discussed in detail in Schröder et al. (2012).
As a consequence, the contact force measurement is now done with additional fibre optic force sensors fixed below the collector strips.
Part of a track measurement with an extraordinarily high density of
events in a track region with many switches. The measured strain data of
identified events are given as point plots in black. Red arrows mark the
events from “hard” hits. “Hard” hits can occur with high and low
intensities. Hits under the level of 200
The contact force
The inertial forces are calculated by
To minimize the measurement uncertainty for
The contact force sensors are made as S-type force sensors (scheme in Fig. 10a, photograph in Fig. 9a).
It is important to understand that FBG sensors, compared to electrical strain
gauges, show different properties when applied across an inhomogeneous strain
distribution. Electrical strain gauges usually integrate the strain
distribution over the measuring area and show a “medium strain” when they
see inhomogeneous strain distribution. In contrast, FBG sensors show a
deformation of the Bragg spectrum, which can lead, depending on the kind of
inhomogeneity and the interrogation algorithm, to significant measurement
errors. In order to avoid a Bragg peak deformation under load, the transducer
has been designed to have homogeneous surface strain in the region of the FBG
under all load conditions. For a vertical point force at distance
Keeping the width constant
Keeping the height constant
A robust construction with constant beam width was prepared by water jet cutting from an aluminium block (Fig. 9a). Fig. 9b shows that there is no deformation of the sensor spectra under load.
With this solution, the variety of materials is very limited because many high-strength materials are available mainly as panel sheets and get their strength from special surface conditioning. Using bending beams from panel sheets, one needs to work with a constant beam height and linear decrease in the width (similar to Fig. 8c). All the following examples work with such constructions.
To achieve a high sensitivity with good reproducibility, it is necessary to
choose a mechanical transducer material with a very high elasticity limit
Rp0.2 (proof stress for maximum 0.2 % strain). We tested spring brass as
well as steel-type NivaFlex
A sensitivity of 14.9
In general, the readout of strain sensors is cross-sensitive to temperature,
and thus a specific concept is necessary for its compensation. As mentioned
above, an anti-symmetric sensor with equal strain responses but opposite sign
of the two single strain sensors
Temperature tests of four sensor heads, consisting of two double FBGs per
sensor head, in an oven with temperatures between 20 and 45
In order to compensate for the measured force values for inertial forces, acceleration sensors are added to the contact force sensor heads.
Different types of FBG-based acceleration sensors can be found in the literature. In Antunes et al. (2011) and Jiang and Yang (2013) an FBG is stretched by a moving inertial mass. In Mita et al. (2001) and Au et al. (2008) a bending beam – or rather a cantilever-based FBG accelerometer – is investigated.
In our work, we developed two different types of cantilever-based FBG accelerometers. Both of them are based on bending beams, which are bent by the reaction of an inertial mass to acceleration. The bending beam has triangular width in order to generate a constant strain distribution (Eq. 6, Fig. 8c). Prototype no. 1 used a spring steel-bending beam with an FBG mounted 2 mm above the surface and a mass of 60 g fixed on the tip of the triangular bending beam.
For the second prototype (no. 2), the mass was fixed by steel springs, and it drives an additional steel triangle with an attached FBG (see Fig. 11c). In this configuration, the mass holder springs and the sensor-bending beam can be optimized separately.
Both types were tested for their sensitivities and resonance frequencies with
the help of a shaker (TIRA S 51128). For prototype no. 1, a sensitivity of
36
Contact force sensor with an acceleration sensor in a test on a
shaker. Detailed descriptions of the experiments are given in the text.
Black: contact force results without acceleration compensation (very low
noise during standstill). Red: contact force results with acceleration
compensation (
To check the performance of the acceleration sensor and to test the
compensation of the inertial force of the force sensor, both types of sensors
were vibrated together in the shaker. The force sensor was loaded with a mass
exciting a gravity force of 15 N. Results of a sample measurement are given
in Fig. 12. In segment 1, the system is undisturbed initially, but fixing the
additional mass excites a strong high-frequency disturbance. This cannot be
compensated for by the acceleration sensor (red curve). However, it is
eliminated after application of a 20 Hz low-pass filter (green curve), as it
is mandatory in European standard EN50317. In segment 2, the mass is applied
and the sensor is not moved. In segment 3, the shaker moved the sensor by a
sinusoidal waveform pattern at a frequency of 10 Hz. The red curve (contact
force with compensation of inertial force) is expected to be smooth in this
section, and these expectations are fulfilled very well. After the mandatory
filtering, a residual deviation from the 15 N force set value was kept
within
The interface between the force sensor and the collector strip was developed by a third party. It fulfils the task of negligible low cross-sensitivities to further forces on the collector strip. The task of reproducible transition of the contact force from the collector strip to the sensor has not yet been solved by this technique; thus, a revision is under investigation.
A first test drive took place on a track of the lignite-fired power plant at Schwarze Pumpe in Germany.
Three sensor heads were mounted below the collector strips (the fourth one was damaged shortly before the tests). The optical signal transmission was achieved by running fibre optic cables directly from the pantograph at 15 kV high voltage to the interrogator in the driver's cabin. To avoid creepage currents on a dirty and wet cable surface, a 20 cm long section of the cable was embedded in a commercially available “creepage path elongation”. A video system monitored the pantograph and the catenary (Fig. 13a) during part of the test drive. From the video, a number of sudden changes in contact force were easily correlated with certain catenary elements (droppers, section insulators, switches, etc.).
This study focused on significant force changes only. However, force changes have been measured relative to a specified reference force value, which was set to the sensor system while the pantograph head was pressed to the catenary and the locomotive halted.
In Fig. 13b, an example is given where the locomotive passed a section
insulator. The event occurred first on the leading collector strip. After a
time delay of 50 ms, the same event was measured by the sensors mounted below
the trailing collector strip. The strips were 60 cm apart and the locomotive
was travelling at a speed of 43 km h
The OCLs run across the track in a zigzag pattern to ensure even wear on the current collector. The zigzag generates a continuous change in the position of the applied contact force on the collector strip. From the difference in the force sensor readings below one collector strip, this OCL position can be calculated. A typical example of a position plot of a right-hand curve is given in Fig. 13c.
Two FBG sensing systems have been developed for monitoring the operational forces at the interface between current collectors and overhead contact lines of electrical railways: (i) fast force changes and impacts in the driving direction were detected by FBG sensors embedded in the collector strip; and (ii) vertical contact forces were detected by FBG-based force measuring heads below the collector strips, also comprising compensation of temperature changes as well as vertical acceleration.
The sensitivity and measurement speed of the sensor system have been proven to be sufficient for this monitoring task. However, the transducer mechanics of the force sensors and the reproducibility of the force results in dependence of dynamic force frequencies require further optimization.
All data are stored at a database at the Leibniz-IPHT Jena (Germany), following the data policies of the institute.
Wolfgang Ecke optimized the interrogator electronics. Kerstin Schröder designed the sensors, planned and performed the measurements, and wrote the manuscript. Manfred Rothhardt helped with discussions about sensor optimization and corrected the manuscript. Uwe Richter and André Sonntag prepared and performed the tests on the locomotives and use the hit detection system regularly. Hartmut Bartelt corrected the manuscript.
The authors declare that they have no conflict of interest.
This work is supported by Freistaat Thüringen (project no. TNA I-1/2012) and co-financed by the European Funds for Regional Development (EFRE), by the AiF ZIM (project no. KF2206912DB3), and by Eurailscout Inspection & Analysis b.v. The authors thank their supporters very much.Edited by: J. Czarske Reviewed by: two anonymous referees