computers in railways vii, c.a. brebbia j.allan, r.j. hill ... · trated. in sec. 4 two controllers...

Active contact force control of pantograph

B. Allotta*, L. Pugi\ A. Rindi* & M. Hindu*

*Dipartimento di Energetica "Sergio Stecco",

University of Flbrence, Italy.

®Scuola Superiore Sant'Anna, Pisa, Italy.

Abstract

In the past 25 years, many researchers have investigated the problem ofoptimum current pick-up in high speed running railways. There is the re-quirement of maintaining almost constant the contact force between pan-tograph and catenary, thus avoiding losses of contact involving mechanical,electric and electromagnetic negative consequences, such as: excessive wear,insufficient current pick-up, high EMI. One of the proposed solutions to thisnroblem is to design servo-actuated pantographs and regulate relevant vari-ables, such as the contact force, during operation. Since 1993, researchers ofthe University of Florence have proposed solutions for the placement of sen-sors and actuators on existing pantographs for high speed operation as wellas their control with the purpose of regulating the contact force at a desiredoptimum level. After the development of simulation models to evaluateperformance and robustness of some control techniques for a servoactuatedpantograph, an experimental campaign is currently going on a symmetricpantograph mounted on a train at rest interacting with a standard 3KVDC overhead line. This paper describes some simulation and experimentalresults obtained with the servoactuated pantograph.

1 Introduction

Current pick-up from line in high speed trains presents problems whichare very well known to the railway community: losses of contact, wear ofpantograph and catenary, electromagnetic pollution and EMI compatibility.Various research groups and companies are currently looking for a solutionto the cited problems [1,2,3]. The researchers of the Department of Ener-

Computers in Railways VII, C.A. Brebbia J.Allan, R.J. Hill, G. Sciutto & S. Sone (Editors) © 2000 WIT Press, www.witpress.com, ISBN 1-85312-826-0

Computers in Railways VII

getics "Sergio Stecco" of the University of Florence have been involves inthis field for many years [4], and proposed to solve the problem by meansof servomechanisms capable of regulating the contact force between pan-tograph and catenary during operation. Till now, the papers publicatedby the group have concerned theoretical aspects of the problem. It hasbeen proposed an innovative configuration of the control which acts in par-allel to the already existing actuators, thus allowing prompt return to thestation in case of failure of the control system. Several numerical simula-tions have been performed to identify the most appropriate control system.Based on these simulations a prototype pantograph has been recently real-ized in collaboration with the ASA Materiale Rotabile e Trazione, ItalianRailways (FS), Florence. The prototype consists in a comercially availablepantograph ATR90 with an actuator on board, capable of exerting to thesliding foots a force devoted to partially compensating the oscillations ofthe contact force caused by the periodic parametric variations encounteredduring operation and by sporadic phenomena (such as defects in line place-ment, aerodynamic effects, etc.). In this paper we describe the prototypeoantograph and the rsults of the first experiments.

The paper is organized as follows: the experimental apparatus used inthe experiments is described in Sec. 2. In Sec. 3 the mathematical modelsused to describe the pantograph/overhead-line and the actuator are illus-trated. In Sec. 4 two controllers are proposed and simulation and experi-mental results are discussed in Sec. 5. Finally, conclusion and future workare presented in Sec. 6.

2 Actuation, sensors and control architecture

2.1 Actuation system

In order to obtain fail safe behaviour, a traditional pantograph equippedwith a cable-actuator capable of regulating the contact force by acting on themain frame or the contact-shoe, has been selected. The existing pneumaticactuator is still used with a higher running pressure so that it gives thenecessary preload to avoid losses of contact. In this way, if a failure ofthe control system or of the cable actuator occurs, the system is able towork also without the servo and the train can go back to the next station.The selected actuator is an electrical actuator and so it has a bandwidthmuch larger than the pneumatic-actuator's one. It would also be possible tochoose a hydraulic actuator, but an actuator of this type isn't an advisablesolution due the necessity of a hydraulic plant mounted on the locomotiveand the consequent problems of maintenance and reliability. Due to theseconsiderations and to the higher cost of a hydraulic prototype compared toan electrical one, an electrical actuator has been chosen.

The chosen actuator is a three-phase brushless motor with sinusoidal


Computers in Railways 111

drive, thus allowing to obtain a very restrained torque ripple. The servo-motor drives, by an epicyclical gear box with velocity ratio of 1:3.34, thedriving pulley on which is wounded up the actuation wire. The motor pro-vides a continuous torque of 2.3 Nm, so that the maximum continuous forceis about 220 N. The peak force is 660 N and it can be kept only for a smalltime. Actuator cable can be connected to the main frame or to the contact-shoe, depending on desired control action.

3.2 Sensors

The pantograph used for the experiments described in this paper is providedof two contact shoes mounted on the main-frame. Two extensimetric load-cells and two accelerometers are set up on each contact-shoe; in this way,assuming that the contact-shoe is a rigid body, it is possible to depuratemeasure of the contact force from the inertial force. For the measurementof the system kinematics, position sensors are also present.

2.3 Control architecture

For carrying out the experiments, a digital controller based on a personalcomputer (Pentium 133 MHz) equipped with an axis control board (MEIPCX-DSP [5]) has been chosen. The main characteristics of the chosen axiscontrol board are:

• 8 ADCs and 4 DACs;

• 4 interfaces for incremental encoder and several digital I/O;

• 1 DSP processor with firmware that is able to realize four simultaneousPID filters on the four controlled axis with velocity and accelerationfeedforward at the frequency of 2 KHz ;

• serial-bus for high speed communication among DSP and interfaces;

• communication port to the bus of the host computer.

The choice of the board is supported by high performances, good versatilityand by relatively low cost. A C-functions library, provided by the controlboard's builder, allows to set up and modify, in real time, the control pa-rameters in the firmware. The C-functions can be recalled by programsrunning on the host.

3 Modeling

3.1 Pantograph and overhead-line model

The lumped model of Pantograph-Catenary's system is shown in Fig. 1 inwhich the time-invariant elements are:



• a lumped mass that represents the equivalent mass of the main frame;

• a constant force that represents the pneumatic actuator acting on themain frame and, running in parallel, a viscous damping;

# a lumped mass, representing the contact-shoe, connected to the mainframe's lumped mass by a spring-damper representing the secondarysuspension;

• a high stiffness with a low damping representing the contact betweenoverhead-line and contact-shoe.

The foregoing data are kindly supplied by the Italian railway's company(FS).

a) b)

Figure 1: Lumped model of Pantograph-overhead line system: a) normal condi-tion; b) loss of contact condition.

Overhead-line's parameters are instead time-varying and we adopted asimply model composed by a mass, a spring and a damper with numericalvalues obtained from experimental data (see [6]) (in fact the changes of suchparameters depends on the pantograph's position along the span).

3.2 Model of the actuator

The actuator is composed by a gearmotor with a pulley mounted on theshaft, driving the command wire. In order to obtain a lifelike model, theactuator has experimentally identified in isometric tests. A wire, of equallength to that mounted on pantograph has been connected to a load cellfixed on the motor's frame; a frequency sweep between 0 and 200 Hz has



been supplied to the motor driven in current mode and, simultaneously,the force acting on the load-cell it has been measured. Using the input'sand output's time-history, acquired by a Ono-Sokki spectrum analyser, theexperimental transfer function of the actuator has been abstracted. In orderto use this result in the simulator, using a 2° order model, a minus-quadapproximation of the transfer function, has been obtained. The used 2°order model is the following:

where r = 0.00197 e f = 0.177.

3.3 The simulator

A simulator of the aystem has been realized, where the two existing contact-shoes are modelled with a single lumped mass with a double numerical value,connected to the main frame's lumped mass with a spring and a damper ofdouble value too.

In the sequel we will refer to the above structure, approximating thebehaviour of the two contact-shoes, with the term "contact-shoe." Themodel allows to execute simulations assuming a) one actuator acting onthe main frame or b) one actuator acting on the contact shoe or c) bothactuators. The use of two different cable actuators presents advantagesfrom the system performance viewpoint, but it involves bigger problemsof reliability in comparison with solutions with a single actuator. For thisreason the configurations named a) and b) above have been investigated. Itseems acclared that solution b) (cable actuator acting on the contact-shoe)is more effective than solution a) [7] and this can be argued by consideringthat the control action is applied closer to the output to be regulated.

In the model the loss of contact between contact-shoe and overhead-lineis properly described. The disturbance due to the locomotive motion is in-troduced by a (known) time variation law of the line mechanical impedance.Span and dropper effects are included in the model as periodical functionswhose frequencies, at a train speed of about 300 Km/h, are 1.4 and 14 Hz,respectively.

4 Control

Simulations have been performed with two different types of regulators act-ing on the main frame because this was the first possible experimental con-figuration. The first one is a PI regulator; the second one is a H^> regulatorallowing to obtain robustness of the control system with respect to modeluncertainties. The regulators have been designed in continuous time andthen they have been discretized, assuming a sampling time of 2 ms. Thediscrete-time versions of the regulators have been obtained by the Tustinbi-linear transformation.



4.1 PI Regulator

PID regulators, (PI is a simplified version), are surely the most used onesin industrial applications due to their semplicity of realization. Moreover,the experimental tuning is facilitated by the direct comprehension of theeffects of the single terms. In the controlled pantograph, the integral term(I) is used to obtain a zero steady-state error while the proportional term(P) is used to increase the system speed of response that, in the case ofsimple integral compensation would be unsatisfactory. In addition to P andI terms, a constant term has been added to partially balance the staticpreload of the pneumatic actuator. The equation of the discretized controlis thus: Uk = u+Kpek + Kj £)i=o( )> where u is the constant term aimedat compensating the preload, KP and Kj are the proportional and integralconstants respectively, e; is the force error computed at time f%, and T isthe safnpling time.x

4.2 #00 regulator

#00 [8] control techniques belong to the wider class of robust control tech-niques. The aim is to maintain stability, performance, disturbance andnoise rejection also in the presence of uncertainties on the linear model ofthe plant (more recently, #00 control techniques have been developed alsofor nonlinear systems). The mathematical background required for the de-sign is huge, but there exists different software packages which simplify thedesigner's job. The design of the controller requires a modified descriptionof the plant (augmented plant), by introducing suitable weight functionsdevoted to penalize, in different manner and depending upon frequency,some tranfer functions among different points of the closed-loop system. Atypical formulation, used in this work for the design of the #00 controller,is the so-called "mixed sensitivity" where the controller searched minimizesthe #oo-norm of the transfer function 2 ^ (optimum problem), defined asfollows:

where W\ and W$ are suitable matrix functions of frequency, 5 is the sensi-tivity function (transfer function between reference input and error), and Tis the closed-loop transfer function between reference input and regulatedoutput. The choice of the weight functions is based on system specifica-tions and (primarily) on designer's experience. This because the solution tothe problem may not exist and the shaping of weight functions may resultdificult and time-consuming. In the case study presented here, the pan-tograph with cable actuator acting on the main frame has been modelledas a SISO system whose control input is the cable tension force and theregulated output is the contact-shoe/overhead-line interaction force. In thenominal plant, average values of line impedance parameters have been used.


Computers in Railways VII 699

The weight functions used are shown in Fig. 2.a, whether graphs of the costfunction T , of the sensitivity 5, of the input/output transfer functionT and the step response of the nominal system are shown in Fig. 2.b. Thechoice of weight functions corresponds to a specification of maximum sen-sitivity equal to 1.2 and a bandwidth of the closed-loop system equal to100 rad/s.

Co«Fufi*bn:Tyiui

Figure 2: HOC controller: a) used weight functions; b) obtained performance forthe nominal system.

5 Results

5.1 Simulations

In this section we show the results of a simulation performed with the H^controller. The contact force setpoint is 180 N. During the first 5 s of sim-ulation only the pneumatic actuator works (in open-loop) with a workingpressure adequate to obtain an average value of the contact force close tothe setpoint. At time 5 s, the supply pressure of the pneumatic actuator isincreased so as to obtain a static force of about 400 N and the controllerof the cable actuator is enabled. The simulation has been performed twice:the first time with a train speed of 200 Km/h and the second one with atrain speed of 300 Km/h. As it can be seen in Fig. 3, the steady statebehaviour is satisfactory, with good rejection of the fundamental frequencyof the disturbance. No simulation results using the PI controller are shown,since they are very similar to the experimental results shown in the nextsection.


700 Computers in Railways VII

a) b)

Figure 3: Simulations performed with the HOQ controller: a) train velocity equalto 200 Km/h b) train velocity equal to 300 Km/h

5.2 Experiments

No real experiments with the #00 controller have been performed yet. Herewe describe some experimental results obtained with the PI controller. Thefirst test reported shows the step response of the system, obtained in thefollowing way: the pneumatic actuator supply pressure and the cable ac-tuator tension have been set to 0.55 Mpa and 100 N respectively, resultingin a certain initial value of the contact force. After 5 s, uhe PI controllerhas been started with gains Kp = 4.0 and Kj = 3.0, and a reference valueof the contact force different from the initial one, has been given to thecontroller as a setpoint. The results of this test are visible in Fig. 4. The

a) b)

Figure 4: Step response with a contact force setpoint of 150 N, using KP = 4and Kj = 3. a) contact force; b) control force.

steady-state accuracy of the servo is acceptable, although the system showsexcessive oscillations during the transient. For this reason the gains havebeen reduced to Kp = 2 and Kj — 1.8. With these new values of the gains,a second test has been performed to evaluate the rejection of a periodic


Computers in Railways 111 701

disturbance. The disturbance is a periodic imposed displacement of a pointof the line placed at about 4 m from the contact-shoe, with a frequency ofabout 1 Hz and an amplitude of about 40 mm (1 Hz corresponds to a trainvelocity of about 216 Km/h). This second experiment has been executedtwice, with different values of the setpoint, and the results are shown inFigg. 5.

a)

c) d)

Figure 5: Rejection of a periodic disturbance with amplitude 40 mm pp usingKp=2.0 and ,K/=1.8. a) contact force in an experiment with force setpoint of150 N; b) control force. The standard deviation of the contact force is reducedfrom 31.2 N to 17.6 N. The average contact force is 150.8 N. c) contact force witha setpoint of 210 N; d) control force. The standard deviation of the contact forceis reduced from 25.9 N a 15.6 N. The average contact force is 209.8 N.

6 Conclusion and future work

In this work a controlled pantograph with cable actuator has been proposed.From the experimental results we can see that the PI controller presents acertain effect of disturbance rejection, although non satisfactory in compar-ision with the rejection of the H one as promised by simulations. In thesame way as we have observed in simulations, we expect that the H<x> con-troller's performances and, particurarly, the robustness will be much better



than the Pi's ones. So we argue that traditional pantographs controlledby wire are an interesting solution to the problem of current pickup. Theaccuracy and the disturbance rejection of the controlled system are com-bined with the reliability of the traditional system. The work will go onwith the test of the H controller acting on the main frame. Afterwards,the solution with cable acting on the contact-shoe will be investigated.

Acknowledgements

The authors wish to thank the Italian Railways Company - FS (Ferroviedello Stato) for supplying the locomotive with the sensorized pantographused in the experiments. The collaborative help of the personnel of theUTMR (Unita Tecnica Materiale Rotabile), FS, Florence is also gratefullyacknowledged.

References

[1] Poetsch, G., Ewans,J. Meisinger, R., Kortom, W., Baldauf, W., Veitl,A., Wallaschek, J., "Pantograph/catenary dynamics and control," Ve-hicle System Dynamics, (5):159~195, 1997.

[2] Kobayasi, T., Fujiasi, Y., Tsuburaya, T, Satoh, J., Oura, Y., Fuji, Y.,"Current collecting performance of overhead contact line-pantographsystem at 425 Km/h," Electrical Engineering in Japan, 124(3):73-81,1998.

[3] Lesser, M., Karlsson, L., Drugge, L., "An interactive model of apantograph-catenary system," Vehicle System Dynamics Supplement,(25) :397-412,1996.

[4] Galeotti, G., Galanti M., Magrini S., Toni, P., "Servoactuated railwaypantograph for high-speed running with constant contact force," Pro-ceedings of the IMechE, part F - Journal of Rail and Rapid Transit,207:37-49, 1993.

[5] Motion Engineering Incorporated "PCX/DSP Motion Control Board:Installation Manual, Firmware version 2.4e," 1996.

[6] Pascucci, R. "Movimenti delle condutture di contatto delle linee fer-roviarie elettrificate," Ingegneria Ferroviaria, 1, 1967.

[7] Balestrino, A., Bruno, O., Landi, A., "Active control for the pantograph-catenary system," Computer in Railways: Railway Design and Man-agement - Computational Mechanics Publications, 1:277-284, 1994.

[8] Doyle, J.C., Glover, K., Khargonekar, P.P., Francis, B.A., "State-space solutions to standard #2 and H control problems," IEEETransactions on Automatic Control, 34(8)-.831-846,1989.


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