RICERCA ITALIANA     

IMPROVEMENT OF CURRENT TRANSMISSION FOR HIGH SPEED TRAINS BY MEANS OF ACTIVE PANTOGRAPHS AND NON-INVASIVE DIAGNOSTICS

Università di Pisa

Abstract

In case of high-speed trains, a regular current transmission in pantograph-catenary systems is a critical task. A regular current transmission in railways is more and more relevant because of European Standards about interoperability: they define the rules for an efficient transnational circulation of high-speed trains in Europe, i.e., a locomotive must be designed to freely operate inside European countries independently from the feeding voltage and frequency. A high quality current collection is characterised by a continuous contact between the pantograph and the overhead contact wire. A poor contact produces various drawbacks, including bursts of arcing: if they have a long duration, efficiency of the locomotive may reduce and an excessive wear of the pantograph strips and of the contact wire may be produced. One of the proposed solutions for achieving a high quality contact is to design servo-actuated pantographs with a control system able to regulate some relevant variables, such as the contact force, during operation. Although the use of a totally active pantograph is advantageous from the viewpoint of potential performance, the reliability of the current pickup would be strongly related to the correct operation of the control system. There exists a hybrid fail-safe solution consisting of adding some sensors and actuators to a conventional pantograph allowing non-optimal operation also in case of failure of the control system. This solution would also have the advantage of a better acceptability for the railway companies and may represent an intermediate step towards the future adoption of totally active pantographs.
The specific focus of the research proposed is the development of a pantograph with new characteristics concerning the materials and the sensors, and the monitoring of its performance, e.g., using IR cameras.
The aim of the project is to join together the different experiences (and the different laboratory facilities) of the research units to develop the following activities:
a) Numerical simulations of the dynamic behavior of the pantograph-catenary system. Analysis of active control and contact force estimation techniques.
b) Identification and testing of innovative materials for contact strips, optimisation of the kinematic parameters of the main frame affecting the dynamic behaviour, study and selection of the algorithms for IR image processing, digital implementation on microprocessors of the designed control and estimator schemes, evaluation of the influence of train speed on aerodynamic forces acting on the pantograph, development of optic fiber sensors for the analysis of the influence of collector strip flexibility on the contact force variations.
c) Experimental tests on the innovative contact strips using the dynamic test-bench (Real-Time Catenary Hybrid Simulator) at Milano unit. Characterisation of the dynamic behavior of the structures also for configurations with different active control techniques and contact force observers. Experimental measurements of the collector strip temperature using an IR camera installed at the Current Collection Test-Rig, to characterise the quality of the collector strip with respect to sparking and bursts of arcing. Correlation of IR images with the measurements of the contact forces provided by dynamometric collector strips, constituting a completely innovative research activity for railway applications.
d) Evaluation of the effectiveness of the proposed solutions from research engineers of the Italian Railways - Unit of Technologies and Rolling Stocks of Florence -UTMR. Such collaboration, already tested in previous research projects, will provide a valuable contribution in validating the proposed solutions under an actual applicative point of view. <<<

Principal Investigator

Alberto LANDI Università di PISA

Research Objectives

The development of innovative methods for improving efficiency, reliability and safety in railway system is a primary objective of the railway research. In case of high-speed trains, a regular current transmission in pantograph-catenary systems is a critical task. A regular current transmission in railways is more and more relevant because of European Standards about interoperability: they define the rules for an efficient transnational circulation of high-speed trains in Europe, i.e., a locomotive must be designed to freely operate inside European countries independently from the feeding voltage and frequency. A high quality current collection is characterised by a continuous contact between the pantograph and the overhead contact wire. A poor contact produces various drawbacks, including bursts of arcing: if they have a long duration, efficiency of the locomotive may reduce and an excessive wear of the pantograph strips and of the contact wire may be produced. One of the proposed solutions for achieving a high quality contact is to design servo-actuated pantographs with a control system able to regulate some relevant variables, such as the contact force, during operation. Although the use of a totally active pantograph is advantageous from the viewpoint of potential performance, the reliability of the current pickup would be strongly related to the correct operation of the control system. There exists a hybrid fail-safe solution consisting of adding some sensors and actuators to a conventional pantograph allowing non-optimal operation also in case of failure of the control system. This solution would also have the advantage of a better acceptability for the railway companies and may represent an intermediate step towards the future adoption of totally active pantographs.
The specific focus of the research proposed is the development of a pantograph with innovative characteristics concerning the materials and the sensors, and the monitoring of its performance, e.g., using IR cameras. Consider now more in detail the tasks of the Project. The activities carried out by the Milan Research Unit have the task to develop a pantograph collector-head with innovative characteristics concerning the materials and the sensors, in order to achieve a regular current collection up to 300 km/h speeds. Florence Research Unit proposes to study asymmetric main frames of railway pantographs with limited encumbrance. Such a limited encumbrance is required to obtain pantographs suitable for multi-tension locomotives and electro-trains with more pantographs on the roof in order cope with different electrification standards existing in European countries. Such study will integrate the activity of the Milan unit, investigating on the upper part of the pantograph: from a strict cooperation between these two groups the complete structure of the new pantograph will be defined.
Together with the study on the mechanical structure of the new pantograph, another research activity will develop control techniques with pneumatic actuation aimed at regulating the contact force between pantograph and overhead line and rejecting the span disturbance. This objective will be pursued mainly from the group of Cagliari, experienced in designing variable structure controllers (VSC). Catania research unit will investigate on techniques to estimate the contact force without the use of strain gauges that are severely affected by the bad environmental conditions in which the pantograph operates. Moreover Catania unit will consider aspects of diagnosis and monitoring of the contact line: in particular a diagnostic strategy and procedure is proposed, based on the use of the Extended Kalman Filter. The idea is to identify typical contact line wear problems or relevant equipment failure, by means of the contact line dynamic relevant parameters, changing from nominal conditions. In particular using simple reference model of the over-all system and vibration measurements referred to pantograph only, a "real time" diagnostic procedure could be defined; the possibility to have on-line suggestion for maintenance purposes reveals very attractive in order to reduce costs for railway companies.
The target of the Pisa Research Unit is to check the validity of new pantographs using IR sensors. A relevant consideration is that this activity will be advantaged from the availability of a professional IR camera and from a test rig emulator of the catenary (Milan unit). Interesting results are hoped and foreseen for a quantitative characterization of new strip materials in terms of distribution of temperature in the presence of break arcs and correlation with wearing.
A different hoped result will be the correlation of the thermal analysis with the measured contact force exerted from the strips: to our knowledge, this analysis is totally new. A possible way for validate thermal analysis results is to have the opportunity for an on-line testing during run tests: it would be meaningful to verify eventual differences of behavior between the tests with IR camera carried out to the bench and on-line tests, at least in case of traditional pantographs.
The aim of the project is to join together the different experiences (and the different laboratory facilities) of the research units, all of them experienced in railway research. Universities of Pisa, Florence and Polytecnic of Milan are share-holders of a company named ITALCERTIFER, belonging to the holding Italian Railways. This project is an occasion to finalize the academic know-how of the partners in order to obtain results of industrial relevance, thanks to synergies with research activities supported by ITALCERTIFER, towards the goal of designing, building and operating reliable current collection systems for high speed trains. The five groups are all very well-known in the international scientific community for their researches on current collection and they represent an excellent team in Italy for addressing the problem at hand. It must be noted that test bench activities are foreseen in this Project, for the availability of dynamic test-bench (Real-Time Catenary Hybrid Simulator) and Current Collection Test-Rig at laboratories of the Milan unit and a high quality IR camera available from the Pisa unit. An evaluation of the effectiveness of the proposed solutions is planned from research engineers of the Italian Railways - Unit of Technologies and Rolling Stocks of Florence -UTMR. The relationships of mutual esteem and cooperation between researchers from all units and UTMR railway engineers', represent an important value added to the project, because it guarantees a technical verification of the results, as it happened in past research activities. <<<

First Results

TASK 1 (Florence): one or more sets of kinematic parameters which define in detail the main frame mechanism. Technical report.

TASK 2 (Milan): definition of the pantograph-collector head.Technical report.

TASK 3 (Pisa): Definition of suitable parameters and of image processing techniques for IR camera. Technical report.

TASK 4 (Catania): Setup of a test-rig able to reproduce the dynamic behaviour of the pantograph-catenary system : control in the frequency range corresponding to the train span passage frequency. Technical report.

TASK 5 (Cagliari): definition of the control algorithms.Technical report.TASK 1 (Florence): Aerodynamic optimisation of the main frame. Technical report.

TASK 2A (Milan): experimental testing of new pantograph collector-heads and comparison with standard solutions,

TASK 2b: construction of a dynamometric collector strip. Technical report.

TASK 3 (Pisa): Definition and testing of algorithms able to processing IR images for diagnostic purposes. Technical report.

TASK 4 (Catania): Setup of a test-rig able to reproduce the dynamic behaviour of the pantograph-catenary system : control in the frequency range corresponding to the train passage under droppers. Technical report.

TASK 5 (Cagliari): improvement of the control algorithms and their implementation on a microprocessor. Technical report.TASK 1a (Florence): Preliminary design of one of more complete pantograph system

TASK 1b (Florence): pantograph design with pneumatic actuation and active control of contact force
Technical report and scientific publications

TASK 2a (Milan): laboratory tuning of the main frame control system at the Milano test rig and verification of expected
performances. Technical report and scientific publications

TASK 3 (Pisa): Correlation of mechanical and thermo graphical data concerning the contact force. Technical report and scientific publications

TASK 4 (Catania): Monitoring of the pantograph-catenary-interaction by means of vibrational measurements and Kalman filtering of data collected during normal train operation. Technical report and scientific publications

TASK 5 (Cagliari): Improvement of the control algorithms using the estimated contact force. Technical report and scientific publications <<<

Timescale

24 months

National and international background

The quality of current transmission has a basic role especially for high-speed trains, since it represents the main limiting factor for the maximum speed; this aspect becomes more critical when the presence of more than one pantograph is required for traction. Current collection quality leads also to important consequences on the wear of the materials of the sliding contact (i.e., the contact wire from the catenary and the contact strip for the pantograph), affecting the maintenance costs and the life-time of the line: the lifetime of a over-head line might decrease from 20-25 years, to less than 10 years with irregular current collection, which often occurs for speeds higher than 200 km/h. At high-speed current collection from overhead line presents problems, well known to the railway companies, because of the loosing of a continuous contact between pantograph strips and the overhead line. Losses of contact involve mechanical, electric and electromagnetic negative consequences, such as excessive wear of pantograph and catenary, insufficient current collection, electromagnetic pollution and EMI compatibility. Usually a static load is applied to the contact strip in order to push it towards the catenary. Moreover, the resultant of the aerodynamics forces usually sums up with the static load and their value is increasing with speed. The contact force presents oscillations around the static value (i.e., its mean value) due to the dynamic interaction between the pantograph and the overhead line. Variations of the contact force lead to several inconveniences in the current collection quality, causing sparking and bursts of arcing in case of too low contact forces. On the other hand an excessive increasing of the static loads produces higher wear rates on the materials for mechanical abrasion. The frequency content of the contact force can be subdivided into three main intervals:
- around 1 Hz: this field is associated with the passage under the single span interacting with the motion of the whole pantograph.
- 1-20 Hz: these frequencies are associated with the passage under droppers and mainly affect the collector strip (or the couple of collector strip plus the collector head);
- 30-500 Hz: this frequency range is associated with the contact strip flexibility, mainly as far as the bending modes is concerned.
An active control system might be developed in order to compensate the contact forces variations for the first two ranges of frequency, up to 20 Hz; several solutions have been investigated and proposed also by the researchers of this project. As far as the 30-500 Hz frequency range is concerned, which is strictly related with sparking and arcing phenomena, the same strategy can not be followed: in fact the required response time of an ideal control system should be extremely fast and the control signals would be affected by an excessive level of electrical noise. Different solutions for achieving a regular current transmission involve a new design of the overhead system: such an approach is only alleviating the problem and it is too expensive for practical applications especially in the case of a modification of existing railway systems. A modification of the pantograph structure has the advantage to be less expensive and more efficient with respect to designing new overhead lines: this research project is based on such considerations. Two research lines are possible: the first one is to design innovative structures and to test new materials for improving current transmission; the second one is to implement active controls. Consider now the state of the art in case of the first research line. Mass decrease and higher levels of structural damping seem to represent an appropriate way to reduce the contact force variations also in the mid-high frequency range. Thus the research should be focused on the materials composing the contact strip. The collector strip (composed by contact strip and its holder) carries out a critical role among the several elements of the pantograph, involved in the current collection problem, since it is in direct contact with the over-head feeding line; as a consequence, it undergoes the maximum levels of acceleration (also up to 100 g) in the whole frequency range of the contact force. Many researches concerning the collector strips are related to a reduction of the mass and an improvement of the shape from the aerodynamic point of view, through experiences in wind tunnel. The simulation of the dynamic behaviour of the pantograph-catenary system by means of performance analysis of the total system, can help the designer to estimate the optimal parameters of innovative pantograph structures, in order to improve the current transmission. Moreover, it is necessary to set up a measurement system of the contact forces to be used during the on-line tests, adaptable to various types of collector strip. The actual methods are based on the measurement of the contact forces through load cells interposed between collector strip and its suspension; they require to compensate for the inertia forces of the system by means of the measurement of acceleration values at the same points. This methods are able to compensate only the single vertical rigid motion of the collector strip, and, for this reason, they are suitable only for frequencies up to 15-20 Hz. Consequently they are unable to take into account the flexural modes of the collector strip, related to the presence of the continuous flashing, source of electro-magnetical disturbances, irregularities of feeding and accelerated wear for combined tribologic and electro-mechanical effects. These last ones have been studied through laboratory tests, performed by several researchers. Currently collector strips are realized with materials and dimensions designed for the d.c. current collection (consequently with high current intensity), or for a.c. one (with smaller current intensity). For example, in case of d.c. curret transmission the contact strips used by Italian Railways are made up of copper or of copper-zirconium alloy, or they are build in Kasperowski configuration, i.e.:, an external copper covering, containing an inner volume of graphite. In the case of a.c.current transmission, instead, the contact strip and its holder are glued togheter, being the contact strip realized in graphite, eventually sintered with copper powder mounted over an aluminium-extruded profile. In the first case the mass generally is between 6 and 10 kg, while in the second case it is approximately equal to 3 kg. Such configurations are characterized by high values of mass and, above all, by a very low damping factor; this in particular for the case of the d.c., whose contact strips are characterised by a value of mass that is so high to constitute an objective limit for running to speeds up to 270-280 km/h, values that should be reached, potentially, also by trains in 3 kV configuration. It is possible to obtain enormous advantages by the use of the graphite: this is a very good solution from the point of view of the over-head line wear, but it suffers from large thermal problems when applied to the 3 kV d.c. trains, requiring up to 1000 A in order to satisfy the maximum power requests. On the other end, the use of copper based alloys, that is commonly adopted on the d.c. trains, is characterised by high wear rates, above all for high speeds; for this reason it is planned by Italian Railways to drop it, adopting innovative solutions that should link the good characteristics typical of the graphite, without incurring in the thermal problems. A global approach to the optimisation problem of the design of the collector strips should need the use of innovative materials, to obtain the following advantages: lower mass value, increase of the structural damping factor, high thermal resistance characteristics and a good connection with graphite contact strips. It is expected that metal matrix composite materials, that has been developed in the field of innovative materials for different applications (aeronautical and automotive) could be a solution. A different line of research for improving a regular current collection is to design servo-actuated pantographs with a control system capable to regulate some critical variables, e.g., the contact force, during operation. Although the use of a totally active pantograph is certainly advantageous from the viewpoint of potential performance, the reliability of the current pickup would be strongly related to the correct operation of the control system. There exists a hybrid fail-safe solution, consisting of adding some sensors and actuators to a conventional pantograph, such that non-optimal operations are guaranteed also in case of failure of the control system. This solution would also have the advantage of a better acceptability in the railway community and may represent an intermediate step towards the future adoption of totally active pantographs.
One of the tasks of the project is to build a prototype pantograph close to the real industrial application. For this reason some interesting, but non-standard solutions presented in literature must leave room, for reliability reasons, to solutions that, although innovative, use more conventional technologies. Accordingly, the use of pneumatic actuators is, e.g., preferred to electric actuators and cable transmission, affected by insulation and mechanical fatigue problems. A correlated aspect is not so considered in literature, i.e.:, the aerodynamic interaction between pantograph and air. This is a relevant study for the influence that the aerodynamic forces may have on the contact force between pantograph and overhead line at high speed.
Several control schemes, based on standard (PID controllers) or advanced (H_inf, frequency-shaping, high-gain and sliding modes) techniques have been presented in the literature. Variable Structure Control (VSC) with Sliding Modes (SM) is a theoretically an effective tool to design non-linear control schemes in the presence of matching uncertainties and disturbances. Nevertheless, its application is not well accepted because of the chattering phenomenon, which is the possible presence of oscillations due to the high but finite frequency switching of the real control device, which could be harmful for resonant mechanical systems. An additional criticism is related to the practical impossibility of having fast switching of the control variable, usually force and/or torques, in mechanical systems. During the last ten years an interesting approach to VSC has gained the attention of researchers. It is based on confining the switching in a higher order derivative of the control variable that, therefore, turns out to be continuous. It is known as the Higher Order Sliding Mode (HOSM) approach to VSC, and it is characterised by a better accuracy.
Many researchers considered innovative pantographs: existing researches on the quality of the current collection are of limited quantity. In Italy the Ministry of Research financed in 2000 the biennial project of research: Innovative Controls in High Speed Transport Systems coordinated by unit of Pisa: the results of this project have been appreciate in international ambit, especially for the contribution in monitoring pantograph-catenary interaction. Such results constitute a meaningful basis for operating unit of Pisa and Cagliari. The main result of Pisa group was the study and implementation of phototube UV sensors: these sensors, mounted on the deck top of a measurement coach, revealed on line their effectiveness in detecting bursts of arcing. A correlation of measured repetitive break arcs with the kilometric progression of the railway line Rome-Florence revealed its validity for an automatic checking of the status of the contact overhead line. These results, validated from a strict cooperation with Trenitalia (Italian Railways) proved to be a useful tool for helping maintenance activities both for the overhead line and for the pantograph. The results obtained are particularly interesting, if compared with the international scenario of innovative research for maintenance: Japanese Railways have projected the Dr Yellow, recently operating, a diagnostic train for maintenance on the Tokyo-Osaka line, whose sensors of arcing are based on UV phototube sensors, just similar to the sensors proposed from the operating unit of Pisa in its previous research activity. As a last remark, it must be highlighted that researchers of this Project have a meaningful basis, as checked from their productivity in terms of the high number of scientific papers published in Journals and Conferences devoted to railway research. <<<