Mechatronic Developments for Railway Vehicles of the Future

IF AC Mechatronic Mechatronic Systems, Systems, Copyright @@ IFAC Darmstadt, Germany, Germany, 2000 Darmstadt,

MECHATRONIC DEVELOPMENTS FOR RAILWAY VEHICLES OF THE FUTURE ~oger Goodall and *Willi *WiIIi Kortiim

"'Department of Electronic and Electrical Engineering, Engineering, +nepartment Loughborough University, Loughborough, LEIl 3TU, UK OK Leicestershire, LE!!

* DLR, Institute of Aeroelasticity, Vehicle System Dynamics, D-82234 Wessling, Germany

Abstract: Railway vehicles have principally been designed by mechanical engineers since railways began in the early 1800s, ie before electronics and feedback control were invented. Today however they contain substantial amounts of electronic and computer control, in particular the traction systems which have been converted entirely. However electronic control can also be applied to the vehicle suspension and guidance functions, which can provide large improvements in performance. More significantly, incorporation of sensors, controllers and actuators into the design process from the start can enable vehicle designers to take advantage of different mechanical configurations which are not possible with a purely mechanical approach - in other words the true spirit of mechatronics and the paper reviews the concepts, the current state-of-the-art and future opportunities. Copyright@2000IFAC Copyright (i) 2000 IFAC suspensio~ control Keywords: Railways, dynamics, active vehicle suspension,

I. 1. 1.1 1.1

straightforward. To this end it is necessary to make use of new lightweight designs and straightforward mechanical configurations. An important possibility for achieving this is by making widespread use of advanced electronic control apparatus, which needs to be embedded within the vehicle system from the . earliest stages of development.

INTRODUCTION

Background

The steadily increasing pressure of competition is forcing the world's railways to reflect economic criteria in planning procedures to an ever greater extent. This begins with maintenance for the track infrastructure and ends with procurement costs for new vehicles. Today's railway systems are in many respects very cost-intensive and hence often uncompetitive in comparison with other modes of transport.

The main systems on a railway vehicle for which a mechatronic approach is appropriate are the suspension, traction and braking systems. Of these, the suspension system is of the greatest interest, partly because traditionally it has been wholly mechanical, but also because it is more fundamental to the complete vehicle design than either of the other two. The main emphasis will therefore be upon the use of active control in suspensions, although some

The rail vehicles of tomorrow must therefore be more cost-effective and energy-efficient. This means that they need to be lighter and mechanically more

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conunents have also been included about the mechatronic state-of-the-art in these other areas.

system, but re-designing the mechanical system to take full advantage of control. So, what are the possibilities for Mechatronics in rail vehicles?

The frrst fIrst 125 years of railway suspension development were characterised by a largely empirical approach to suspension design. The bogie has been invented and patented in 1809, but it took some 40 years to become regularly used in Europe after the beginning of public railways, although it was adopted rather more quickly in the United States. This "Empirical Design Period" extended throughout the frrst fIrst half of the 20th Century, and some very successful suspension systems were in this way designed. designed. The use of the word "empirical" is not suggesting that no calculations were carried out in the design, rather that the parameters which determined the suspension's fundamental performance in terms of stability, curving, and ride quality were derived from experience and experiment.

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/ - of Mcchatronics?

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Fig. 1 Evolution in vehicle design Fig. This can best be answered by considering the way in which rail vehicle design has evolved historically l. In the early days of railways, the vehicles see Fig. Fig. 1. were mechanically very simple. They were, essentially, boxes on two sets of wheels. At the time passengers thought they were wonderful, but in fact their performance was poor. Demands for higher speed and better ride quality necessitated evolutionary development over a period of 150 years or more, leading to the basic structure of high speed passenger vehicles which we have today - four-axled vehicles with two bogies which can run stably at high speeds and go round the curves in a reasonable manner, and having soft secondary suspensions to provide modem standards of ride quality. It is however clear that these are heavy and mechanically complicated, and one of the best ways of achieving lighter, simpler vehicles is to go back to the structure of those early vehicles. vehicles.

In the 1960s the science of vehicle dynamics came properly of age, and what can be thought of as the "Analytical Design Period" began. Although some of the analytical ingredients had been known since the early 1900s, it was not until the 1960s that all the bits and pieces were put together and effective analysis became possible (Wickens 1998, Gilchrist 1998). At the same time computers became available to analyse and predict the perfonnance performance of complete railway vehicles, which are still one of the most complex dynamic systems in engineering and without computer analysis would remain intractable to analyse. There had of course been a number of fascinating, ingenious and highly intuitive designs prior to this time, but it is notable that the new understanding has enabled much higher operating speeds than had previously been thought possible, and has provided mechanical design innovations which are based upon a fundamental re-think of suspension design - cross-bracing, steering bogies, single axle bogies, etc.

This is something which most people agree cannot be achieved effectively with passive suspension technology, but can be achieved through active control. The lower weight and mechanical complexity reduce both capital cost and running cost, and this can be spent on the provision of the active systems and their maintenance. The challenge is to design the active systems so that there is a net benefit. benefIt.

The arrival of active suspensions has heralded the "Mechatronic Design Period", although it will be seen that there is still a long way to go in terms of fulfilling its promise. fulfIlling promise. It is worth remarking that there have been a large number of theoretical studies, but the emphasis in this survey is upon practical implementations. Section 4 of the paper will deal with future possibilities and will include some ideas which so far have only been explored theoretically, but which might point the way to the future. future .

1.2

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This principle, of simplifying the mechanical system by the use of active steering and active suspensions, in other words exchanging mechanical complexity for electronic control complexity, is potentially an extremely important contribution to railway technology, but there are major technical challenges to be overcome. The following sections of the paper will identify what has been achieved so far and what challenges remain.

Application of mechatronics 1.3

If it is assumed that there will be actuators, sensors and controllers at the heart of future railway vehicles, it is possible to discover ways of exploiting the synergy of the mechanics and the electronics to achieve a superior solution. This is of course what the discipline of Mechatronics is about, not just adding electronic control to an existing mechanical

Research objectives

One fundamental problem with active vehicle systems is the interaction between structural dynamics, the forces between wheel and rail, and the control systems. Accordingly, a proper understanding of the engineering science is needed for an integrated 22

application of electronic and mechanical components to achieve the optimum systems design of railway trains, and it is possible to identify the following specific objectives: I. 1.

2.

33..

which in principle has 6 degrees-of-freedom (ie 3 th translational, 3 rotational), resulting in an 84 order set of differential equations. Fortunately some of these degrees-of-freedom can sometimes be needed: neglected. However the following are still needed: two for each wheelset (lateral and yaw); five for each bogie and the same for the vehicle body - lateral and vertical plus the 3 rotational modes pitch, yaw and roll - the longitudinal motion is essentially the vehicle speed. This reduces the model to 23 degrees of freedom, freedom. but a few extra states are needed to account for flexible modes and for airspring dynamics, so a system of order 50 results. It might be expected that some of the modes are going to be at high frequency and can therefore be neglected, but analysis of the system eigenvalues shows that all but a very few will be under 10Hz, so their dynamic included. effects are all significant and need to be included.

To develop a fundamental understanding of the dynamic response of lightweight rail vehicles with active controls. To develop methods of analysis for advanced vehicles emphasizing configurations which take full advantage of emerging control technology (leading to reduced weight, lower cost, lower car-body structural vibrations, etc.) To research systems architectures (sensors, actuators, processing) which provide the level of safety, reliability, and maintainability needed for an operational railway. 2. RAILWAY RAIL WA Y VEHICLE DESIGN

2.1

The previous paragraph highlights the dynamic complexity, but this is compounded by substantial non-linearities, especially associated with the properties of the wheel-rail contact mechanics, which affect the overall dynamics significantly. significantly.

System dynamics

It's useful to explain the essential mechanical arrangement of a modem high-speed rail vehicle shown in Fig. 2, which is an arrangement which has evolved over nearly two centuries of railway operation. There is the vehicle body for the passengers, and underneath are two bogies each having two pairs of wheels, connected by a solid axle to form what's called a wheelset. The suspension arrangement which interconnects these seven masses - one body, two bogies, four wheelsets - is complicated. There is the primary suspension from the wheelsets to the bogie, mainly dealing with running stability. Also there is the secondary suspension from the bogies to the vehicle body, nowadays usually a very soft set of airsprings which ensure that the passengers experience a good ride quality. Both stages of suspension are carefully quality. designed in the lateral and vertical directions, and some of the rotational modes such as yawing and rolling are important as well. Of course what is shown here is just a "passive" arrangement where the performance depends upon springs, dampers, and so on.

2.2

Control law formulation

Vehicle outputs (accelerollOn (acceleratIOn., displacement, ere) displacement. etc)

Fig. 3 Scheme of an active suspension The general scheme for an active suspension is shown in Fig. 3. This illustrates how feedback loops are added to enclose the dynamic system, the complexity of which affects the formulation of the control law: classical or intuitively-based strategies can be created, but their theoretical performance is confused by the complexity; modem model-based strategies (eg optimal control, H infmity control) can cope with the complexity, but result in very complex controllers which may be practically difficult to implement. Normally of course restricted models will be used for design of specific active suspensions: an end-view model for tilting, a sideview side view model for the vertical suspension, a plan view model for the lateral suspension and for wheelset dynamics. These simplified models are important, but the complexity is still high and there are a number of other higher frequency modes and non-linear effects which need to be included, and so the design problem remains complicated.

Fig. 2 Railway vehicle scheme Quantifying the level of the complexity is instructive. A conventional vehicle has 7 main masses, each of 23

2.3

Design requirements

necessity of a sophisticated model for wheel/rail contact.

Another issue relates to what has to be achieved when designin~ designing an active suspension controller, and ~hen

Because of its specificity, general modelling and simulation do not support railway vehicle modelling. modelling. Only some multibody system (MBS) modelling and simulation packages have implemented the wheel/rail module. Some dedicated packages were also developed. The linear and the simplified nonlinear theories developed by Kalker (FASTSIM), (F ASTSIM), (Kalker 1982), are used for modelling of wheel/rail contact, because of many validation measurements. measurements. It is preferable to simulate directly the contact of the wheel on the rail rather than the contact of the wheelset on the track, e.g. because of independent simulations. wheel sirnulations.

Fig. 4 summanses summarises the design requirements. On the FIg. left are the inputs, and these are separated into three detenninistic track inputs describing the types: the deterministic intended track features - curves, gradient, etc; the random track inputs describing the irregularities, ie the deviations from the intended track position; and thirdly there are the force inputs, principally the changes in load as people get on and off the vehicle. All these occur in the vertical, lateral and roll direction. CeatnUcr

The design process of modem modern railway vehicles is affected by a number of different disciplines influencing each other, such as control, flexibility, aerodynamics, hydraulics, etc. etc. In order to apply these facilities to the model, the packages enabling to simulate the wheel-rail contact must be extended with new design features. For example some MBS packages include simple and restricted control loops; alternatively the controller must be added as a userdefmed element. The connection of MBS or railwaydedicated packages with control design packages is amore convenient way for the designers. The coupling of simulation codes is described e.g. in (Veitl et a1. 1999). Using different specialised packages for mechanical and control design decreases demands on the designer. Despite using different packages, it is possible to simulate and optimise whole vehicle model from the earliest design stages

I V~h'c1~ SYstem I

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Fig. 4 Design requirements On the right are the outputs, and four items are shown. The acceleration levels on the vehicle body, which represents the quality of ride, are to be minimised. The suspension movements must not become too large, and must therefore be constrained. It's also necessary to ensure that there is a minimum margin of stability, another constraint. And fmally there's curving performance, mainly about minimising any wear of the wheels and the rails, so an optimisation is needed. There are therefore three input types to consider, two output measures to minimise and two design constraints to meet, and the diagram also indicates the influences from the inputs to the outputs, the end result of which is a non-trivial multi-objective design process, particularly when combined with the dynamic complexity mentioned earlier.

2.4

3. MECHATRONIC OPPORTUNITIES This section provides an overview of the possibilities that exist for mechatronic railway vehicles. It provides a progressive overview, starting with the active control techniques which are currently employed and moving on through a sequence of opportunities which are increasingly further away from the current state-of-the-art in railway technology.

Methodologies and tools

Railway vehicle design is still dominated by the mechanical engineering aspects, but taking advantage of mechatronic opportunities requires the engineers to take on board not only the new conceptual approach implicit in the discipline, but also the design tools which are required for an integrated approach to mechanical, electronic and software system design.

3.1

Tilting trains

Tilt is increasingly becoming accepted as standard equipment for speeding up trains, and a number of European countries have developed tilting trains: Italy with the Pendolino concept, Sweden with its X2000 trains, Spain with the Talgo, and more recently Germany with the VT 611 multiple units and Switzerland with a concept developed by SIG, (Various authors, 1997). Further afield Bombardier makes a tilting train in N. America, and there are a number of Japanese systems either in operation or at an experimental stage (Sasaki et aI., 1996).

Modelling and simulation of railway vehicles is specific, not only because the model for dynamic analysis should include model of suspension components, which can be modelled by many simulation tools, but particularly because of the

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but this development was not taken any further at the time.

The concept of tilt. tilt. The basic idea is to lean the vehicles inwards on curves to reduce the acceleration felt by the passengers but this acceleration as a vehicle passes onto a railway curve does not rise suddenly. There is a transition from the straight to the curve, usually lasting around 2 seconds, which is a deliberate design feature so that passengers are not made uncomfortable by too sudden an application of sideways acceleration. Normally the track is leaned inward or "canted" to reduce the lateral acceleration experienced by the passengers, and this also Fig. 5. increases steadily through the transition - Fig. Acceler.uion Acceler.ltioo

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Fig. 6 Typical tilting train cross-section

!,=';ved acceler.uion

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Interestingly, the mechanism by which the tilt action is provided is still changing. Fig. 7 shows the crosssections for both the first Italian "Pendolino" trains manufactured by Fiat, and the company's newest design which comes into service in Switzerland in 2000 and UK in 2001 (Ford, 1998). Originally there were large flexicoil springs for the secondary suspension, the hydraulic tilting actuators were mounted vertically in the body of the vehicle, and the pantograph which collects power from the overhead line was connected by a mechanical linkage to the bogie in order to avoid it tilting with the vehicle body and dropping off the wire. In the new scheme there is a big single airspring in the centre, rather than two at the sides which is the usual arrangement. A circular roller beam will be used rather than the inclined link mechanism, and in line with the trend identified in the previous paragraph these vehicles will also use electro-mechanical actuators. Another change is that, in order to avoid the pantograph moving with the tilting body, the mechanical linkage used in the early systems has been replaced with a separate controlled actuator to counteract the tilting, ie much more of a mechatronic solution.

Tilt

Cant

Straight

I Transition l~

Curve

Time

Fig. 5 Tilting acceleration responses At higher speeds the curving acceleration rises, and the transition will also be more severe because the duration of the transition will reduce, that is unless the track is changed. This is where tilt comes in, to bring the acceleration back to the level it was before. However, it's not only what happens in the steady curve that's important, but also the dynamic response during the transition. Ideally the tilt angle of the body should rise progressively, perfectly aligned both with the onset of curving acceleration and the rising cant angle. However there is a complication, because irregularities in the track (ie imperfections between how the track is meant to be and how it actually is laid) have the effect of adding a random higher frequency element to the acceleration perceived by the passengers. Tilting cannot affect this because it's principally a purely lateral effect, and the purpose of the lateral secondary suspension is to keep these accelerations low enough. In fact, the tilt system must avoid reacting to these irregularities, otherwise a number of problems arise. Mechanical schemes for tilt. A typical arrangement is shown in Fig. 6, which is a cross-sectional view of the X2000 train developed in Sweden by ADtranz ai, 1995). The airsprings which form (Anderson et aI, the secondary suspension sit on top of a tilting bolster, connected to the bogie by inclined swing links. Hydraulic actuators fitted between the bogie and the bolster create the tilting action. The inclined swing links mean that the effective tilt centre is somewhere above the vehicle floor level, even though the action is applied below the vehicle body. In Europe there is now a strong trend to use electromechanical actuators, instead of hydraulic actuators which was the normal solution in the early days of tilt. In fact it was recognised that electro-mechanical tilting provided a better system solution almost 20 years ago in the UK (Pennington and Pollard, 1983),

Early Pendolino scheme

New Fiat-SIG scheme

Fig. 7 New tilting train cross-section Tilting controller strategies. The most intuitive control approach is to put an accelerometer on the y to measure the lateral acceleration vehicle hod body which the tilt action is required to reduce. The accelerometer signal is used to drive the actuator in a direction which will bring it towards zero, ie a 25

classical application of negative feedback. There are two problems with \vith this approach. approach. Firstly, if the passengers are left without lateral acceleration, a significant proportion of them experience motion sickness. The other problem is interaction with the sickness. lateral suspension, and it can be shown that if the tilt loop bandwidth is low enough not to interfere with the lateral suspension, it's then too slow-acting on the curve transition.

follow the way in which the track moves vertically as along . The secondary springs from the vehicle travels along. the bogie to the body are there to transmit the low frequency intended movements so the vehicle follows the track, but at the same time to isolate the higher frequency irregularities to provide a good ride quality. quality. The amount of damping provided for the secondary suspension is a difficult design trade-off. If it's too low there will be a lot of activity in the resonant modes; if it's too high the dampers transmit high frequency track movements to the vehicle body, and modal dampings in the region of 20% are typically chosen.

The dynamic interaction problem can be avoided by putting the accelerometer on a non-tilting part, in other words the bogie. This provides a "tilt angle command signal" for a feedback loop using a measurement of the tilt angle, typically scaled so that the system compensates for around 60 to 70% of the effects. curving acceleration to avoid motion sickness effects. However the accelerometer on the bogie is not only measuring the curving acceleration, but also the pure lateral accelerations due to track irregularities. Consequently it's necessary to add an electronic filter to reduce the effects of the irregularities, otherwise on straight track this results in a poor ride quality, but this creates too much delay at the start of the curve.

Use of active elements. Active control provides a solution, because by replacing the dampers with actuators, measuring the vertical velocity at each end of the vehicle and making the actuator force proportional to the body velocity, the actuators then reference, and apply damping to an absolute reference, increased damping now controls the resonance of the suspension without making things worse at high frequencies. frequencies . This is the concept known as "skyhook damping", identified many years ago (Kamopp, 1978) but still providing the basis for most active suspensions. suspensions. Some care in the controller design is needed because skyhook damping can create large suspension deflections when gradients and curves are encountered (Li and Goodall, 1999), but overall the concept is extremely beneficial.

The solution is to use the signal from the vehicle in front to provide precedence, carefully designed so that the delay introduced by the filter compensates for the precedence time corresponding to a vehicle length. And this scheme, albeit with detailed use. variations, is what most manufacturers now use.

This evolution of control strategies took place over nearly 30 years. years. Initially it seemed to be a straightforward control system, and lengthy development time arose a proper mechatronic systems approach which recognised the complete problem "domain" was not adopted - the complexity of the basic dynamic system, the form of the track excitations, the tilting mechanism, the sensing possibilities, etc. Nevertheless it can be seen that there are now clear signs of manufacturers starting to think mechatronically, resulting in more effective, better-integrated designs designs..

3.2

Fig. 8 Active secondary suspension control scheme This implementation of skyhook damping gives an important improvement in ride quality, but in fact there are other things which can be done. For example, normally the suspension frequency in pitching is somewhat higher than in the vertical or bouncing direction, whereas it can readily be demonstrated that there is a defmite advantage in having a lower frequency in pitch, but with an active suspension it is possible to bring together the signals from the two ends of the vehicle, separate them into bouncing and pitching and independently control the vehicle modes, in particular to make the pitching 8. The response significantly softer - see Fig. 8. combination of this modal approach and skyhook damper gives substantial improvements in ride quality, and both theoretical and experimental investigations have shown up to 60% reduction in rms acceleration levels.

Active secondary suspension

The introduction of tilting is potentially the tip of an iceberg: iceberg: once an active system has been introduced and accepted for railway operation, the introduction of other active solutions is technically much easier. Tilting is a specific form of active secondary suspension between the bogie and the body, so the paper next looks at how the idea of active secondary suspensions can be generalised, principally with the objective of improving the vehicle response to track irregularities, ie to improve ride quality.

Passive suspension characteristics. In a railway vehicle the primary suspension from the wheels to the bogies is fairly stiff, the bogies more or less

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Applications. Applications. The only active secondary suspension which has run regularly in service in Europe (apart from tilt) is a low grade active lateral suspension using pneumatic actuators on the Fiat Pendolino trains, and this same facility is provided on some of the newer Fiat tilting bogies seen in Fig. Fig. 7. However there are also a number of successful examples of full scale experimental implementations (Goodall, 1997) which help to understand the mechatronic opportunities.

A very significant signiflcant development has been conducted by Siemens SGP in Austria on an experimental vehicle. vehicle. They have semi-active secondary vertical dampers, fully-active pneumatic actuators to keep the lateral suspension centred on curves, and electromechanical tilt actuators (mentioned before in connection with trends in tilting trains). What is particularly interesting about this development is the way the control systems have been integrated, including a full set of inertial sensors, and this almost certainly represents the most advanced mechatronic implementation which is currently available (Stribersky et aI. aI.,, 1998).

One of the earliest major experimental study was undertaken by British Rail in the early 1980s. This covered both lateral and vertical secondary suspensions using a variety of actuator technologies, and was the frrst flrst practical demonstration that significant improvements in ride quality could be signiflcant achieved, of around 35 % - 50 % during mainline testing (Pollard and Simons, 1983).

Most of the serious experimental work at present is concerned with the lateral direction, because generally speaking that is the harder design problem. problem. It is probable that once these have been proved in operation the deficiencies deflciencies of the vertical suspensions will be highlighted, and this may prompt a spate of work on active vertical suspension. suspension. For example a laboratory rig has been used to assess the use of actively-controlled damping for a secondary air suspension (Tang 1996).

In the early 1990s there was a development by ADtranz in the UK, what they called their "active hold-off device". This was designed to improve ride quality simply by holding the suspension away from the lateral bump stop on curves, which avoided the high transient accelerations due to contact with the bump stops. The system has been tested comprehensively, including evaluation on a service vehicle (AlIen, 1994).

3.3

Controlling wheels and wheelsets

Control of the primary suspension to give active steering/guidance is a much more substantial step in technology, and this sub-section reviews the principles and explains the kind of practical implementations which have been investigated.

ADtranz in Sweden has looked at a different concept - a "semi-active" electronically-controlled lateral damper for use on their X2000 tilting train. train. Other people have studied the use of semi-active devices, but they have usually modified modilled a nonnal normal hydraulic damper, whereas the ABB approach is very novel (and patented) (Roth and Lizell, 1995). Essentially it is a displacement pump driven by the movements of the actuator, and braked by a miniature electronically-controlled brake, an arrangement which seems to achieve a wide range of linear operation, and is a good example of a mechatronic approach at a component level rather for the whole system. Alstom has also looked at semi-active systems in laboratory tests (O'Neil and Wale, 1992), including the assessment of devices using electrorheological fluids, the characteristics of which are directly affected by the application of an electrical voltage.

Wheel flange ~ .............

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Solid axle

Track Tl1Ick

Fig. 9 Features of a railway wheelset

Wheelset dynamics. It's necessary to understand how the conventional railway wheelset works, which has been a vital element of railway vehicles since railways began, but the way it works is not always obvious to people. A wheelset consists of two coned or otherwise profIled proflled wheels rigidly connected by an axle - Fig. 9. On straight track the wheelset runs in a centralised position, but when a curve is encountered the wheelset naturally moves outwards; this causes the outer wheel to run on a larger radius and the inner on a smaller radius. Being connected by the axle the wheels must still rotate at the same rotational speed, so the outer wheel moves faster along the track, and the effect is to make the wheelset go around the curve. A popular misconception is that it is the flange which makes the vehicle follow the curve, but in fact this is not the case - it's entirely a consequence of the profiling proflling of the wheels and the designer's job is to

The Japanese are also studying active suspensions. Hydraulic .and and pneumatic actuators have been tested on their WIN350 train which is being used for research and development aimed at achieving 350 km/h operation for the Shin Kansen trains (Sakurai et aI., 1996). A semi-active lateral damper is used on leading vehicles of the Series 700 Shinkansen trains (Norinao, 1997) to provide a satisfactory ride quality for 300km/h operation. There seems to be a lot of Japanese active suspension work, much of which appears to be closely related. They have a 3 car "TryZ" test train for evaluating 3 different active suspension concepts for conventional trains, i.e. not only for the Shin Kansen (Sasaki et aI., 1996). 27

avoid contact between the flange and the rail otherwise large amounts of wear results.

One possibility is where the longitudinal springs seen in Fig. 10 are replaced by actuators, and this enables characteristics to be obtained which either are not possible with a purely mechanical solution, or at least extremely difficult in practice. In the more general case a wheelset can be actively-connected in both directions, including directly to the vehicle body (ie without a bogie), which provides the basis for identifying a range of possible control approaches. These are outlined in the sections which follow, although it's important to emphasise that these are very much concept diagrams to describe the basic idea, and a lot of extra engineering detail would be needed in practice.

Unfortunately, while this inherent curving action is obviously just what is needed, there is a problem, and this arises when you start to look at the dynamics of the wheelset. Its motion occurs both laterally and in the rotational yaw direction. The forces on the wheelset arise from so-called "creepages" between the wheel and rail, small relative velocities which arise because of elastic deformation of the steel at the point of contact and which apply in both the longitudinal and the lateral directions. The overall effect is an instability, principally a kinematic oscillation (Wickens, 1969). Adding mechanical dampers doesn't stabilise the wheelset, and it is necessary to add springs, and the normal solution is to have two wheelsets connected via longitudinal and lateral springs within a bogie as seen in Fig. 10. The lateral springs are necessary to transmit the curving forces, but the longitudinal springs are mainly there to stabilise the wheelsets, essentially by providing a stiffness. yaw stiffness.

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Fig. 11 Actively-steered wheelset The first option in Fig. 11 shows the longitudinal springs in series with the actuators - the idea here is that the higher frequency oscillations of the wheelset are stabilised by the springs, with low bandwidth active control provided by the actuators, essentially to "relax" the force they produce when the vehicle goes round curves so as to allow the wheelset to take up its natural curving position. This approach has and it's been studied applied to a conventional bogie, a~d been shown possible to get extremely good performance with really quite a simple control law which minimises the wheelset yaw torque applied at low frequencies (Shen and Goodall, 1997). Actuator low, a few tens of Watts power levels are extremely low, only, which means that practical implementation is not a problem. The same idea has also been applied to single-axle running gear in Germany, and forms the basic scheme in some Integral railcars, manufactured in Austria and now running in service, (Sommerer, (Sommerer, 1999).

Fig. 10 Mechanical scheme of a conventional bogie However, on a curve these stabilising longitudinal springs produce forces which interfere with the natural curving action of the wheelset. The result is that on the tighter curves the wheel flange will be in contact with the side of the rail, causing wear of the wheels, wear of the rails and often significant amounts of noise. There is therefore a difficult design trade-off: trade-off: stiff springs give stable high-speed running, but poor curving; soft springs mean that the curving performance is better, but stable running is only possible at low speeds. speeds. Great ingenuity has been applied in fmding mechanical solutions to this design trade-off (Illingworth and Pollard, 1982): 1982): "crossbraced bogies" with carefully-designed linkages between the wheelsets; "steering bogies" with linkages to the vehicle body which try and get the wheelsets into more or less the right position on steady curves, but none of these fundamentally overcomes the problem which has been outlined.

Active control of wheels and wheelsets. It is now possible to discuss where control fits in, in, because appropriate use of active control can provide new levels of performance not achievable with a conventional passive bogie. bogie.

Fig. 12 12 Actively-stabilised wheelset Fig. 12 shows another approach, which is is the other Fig. to the the previous previous paragraph paragraph - passively way round to

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steered, actively stabilised. As mentioned earlier, don ' t stabilise the wheelset; conventional dampers don't however applying a yaw torque to the wheelset which is proportional to the lateral velocity of the wheelset produces a form of active damping which is stabilising, and a detailed analysis shows that it doesn't interfere the natural curving either. In fact a number of control laws are possible for this scheme, not only the active damping principle but other ideas as well (Mei and Goodall, 1999).

(Frederich, 1999), and Fig. 15 shows the wheelset assembly. assembly.

Fig. 15 Torque-controlled running gear

~earings

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Fig. 13 Wheelset with independently-rotating wheels

Stub-Axle

Wheelsets with independently rotating wheels. wheels. A et which railway modification of the basic wheels wheelset suspension designers have looked at many times over the years is wheelsets in which the wheels are free to rotate independently on the axles (Dukkipatti, 1993). It's commonly stated that this removes the instability because the two wheels are no longer connected mechanically. mechanically. In fact if you analyse this carefully this is not the case (Goodall and Li, 2000), but stabilisation is much more straightforward, and in this case dampers are effective. However the natural curving action disappears, again because the connection between the wheels has disappeared, and so active control in this case is there primarily to steer the wheelset through the curves, as Fig. 13 shows. shows.

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Fig 16. Directly-steered wheels

Directly-steered wheels. wheels. So far the paper has looked at applying control to conventional wheelsets, but the next scheme, seen in Fig. 16, is totally different because there is no axle. The wheels are pivotted about a vertical axis, and there is a track rod connecting the wheels to keep them parallel. There are a number of ways in which steering can be achieved - here the steering angle is directly adjusted by an actuator, but differential torque control is another choice (Powell and Wickens, 1995) ie the idea described in the previous paragraph but applied via a different mechanism. mechanism. The wheels are of course now completely independent and must therefore be steered. Here we can see significant mechatronic influences starting to appear, because the basic mechanical scheme has been fundamentally changed to accommodate the benefits provided by active control.

Fig. 14 Torque-controlled independently-rotating wheels

Range of options for active wheellwheelset control. The preceding sub-sections have indicated a great variety of possibilities for achieving active steering. Firstly, there is the control approach - is the control being used to steer to stabilise or both? Secondly what method is providing the control - actuator control or torque control? Also to be decided is the wheel configuration normal solid-axle, independently-rotating wheels on an axle, or steered wheels? Superimposed on these choices is also the

Fig. 14 presents an important alternative idea which can be used with independently-rotating wheels rather than use actuators, it is possible to control the torque being applied to the two wheels separately, the sum of the torques providing propulsion and braking as normal, with the difference being used to steer the wheelset. This idea has been investigated in Germany and demonstrated on a laboratory rig (Gretzschel and Bose, 1999). Also in Germany there is an experimental vehicle which uses just this approach 29

type of control hardware - sensors, controllers actuators, etc - and so the decision as to which is the best solution for a particular application is complex and still open, and it is clear that significant research is still needed to help resolve some of these issues. issues.

3.4

steering. As mentioned earlier, the possibility has already been recognised of avoiding the need for steering actuators if the traction motors driving the wheels are differentially controlled to achieve the steering action. Obviously this needs a high level of integration between the suspension and drive control to accommodate the systems, particularly requirements of safety criticality. An exciting new concept is the idea of a "wheel motor" in which there is mechanical integration of the wheel and the traction motor (Shelly, 1999) - see Fig 17.

Traction and braking

Electric traction systems have been common in railways for many years, and the addition of electronics and control has mainly been associated with exploitation of power electronic drives. Probably the main consequence of this has been the ability to replace d.c. motors with smaller and more efficient a.c. motors. motors. In general this has resulted in an easing of the mechanical design, rather than the kind of fundamental change which follows from mechatronic integration, although there have been cases of vehicles for low speed systems in which "hub" motors have been directly connected to the wheels, thereby removing the need for a gearbox and/or mechanical drive and creating substantial simplifications in bogie design. The other significant impact is that the traction system has been able increasingly to provide braking effort through electrical regenerative action, although the net effect upon the main mechanical braking system has so far been relatively small.

Fig. 17 Wheel motor Fig. It must be remembered that the suspension, guidance, propulsion and braking forces of a railway vehicle all pass through the contact point between the wheels and the rail, a patch of compressed metal perhaps 2cm in size. There is a tendency to design these systems independently, but once they are all electronically-controlled, it will be possible to optirnise the use of this contact patch through an optimise integrated control system, and Fig. Fig. 18 shows an overall scheme. There will be a set of sensors measuring what's happening, a controller which carries out drive, suspension and steering control, and the forces produced all act upon the vehicle dynamic system. The diagram also shows a track database system which could contain the intended alignment aligrunent of the track, although important questions arise information. regarding the accuracy of the track infonnation. Nevertheless, this kind of conceptual approach raises possibilities. some extremely interesting possibilities.

Advanced control can provide a much more effective use of the adhesion which is available at the wheel/rail interface, and there are examples of theoretical and experimental studies which demonstrate the ability through the sophisticated power control to operate at the peak of the adhesion/slip characteristic, (Beck and Engel, 1996), (Schwartz and Pfeiffer, 2000). 2000). Wheelslide protection of railway braking systems has been fitted for many years, but the pneumatic brake actuators which are predominant in the industry are insufficiently fast to do other than provide fairly simple on-off control when wheelslide is detected. Recent examples of wheelslide protection are making much more extensive use of the sophistication which is possible through electronic control (Anon, 2000). In addition, a number of braking system manufacturers are investigating more advanced braking actuators (e.g. (e.g. electro-mechanical systems), at least in part because of the enhanced control capability which these will provide, and in many senses these developments reflect the changes away from traditional mechanical engineering solutions which have also been seen with the suspension systems and towards a mechatronic approach.

Fig. 18 Integrated control scheme

4. FUTURE FUTIJRE MECHATRONIC MECHA TRONIC VEHICLES

So it is possible to look forward to a full implementation of mechatronic principles for the design of running gear, using single wheels in which electronics is applied to an integrated control system, controlling traction, braking and suspension forces in

As mechatronic trains develop more and more electronics will appear within railway vehicles, and more profound integration of the various systems is possible. The propulsion system already has a lot of electronics, and this can be linked in with active 30

motivated by substantial changes in vehicle configuration. The engineering challenges are greater, but the operational benefits are likely to be substantial if safe, reliable, cost-effective solutions can be developed.

an optimal way. The concept, in particular the use of fron1 the active steering, is potentially liberating from operational viewpoint, because in principle it is possible to dictate the direction through switches from the vehicle rather than the track. Conventional track switches could disappear such that the track is continuous in both directions through the switch, with the sensors causing the vehicle to follow one route or the other, and if this is combined with electronically rather than mechanically-coupled vehicles all sorts of possibilities arise which could significantly enhance the operational flexibility of railway operation.

It seems inevitable that rail vehicles will become "increasingly mechatronic" in some form fonn or another, and comparison with the other industries mentioned makes it clear that such an evolution (or revolution?) is strategically important for railways. An EC research project, in which both the authors are involved, is making an important contribution to the evolution of this new mechatronic technology for trains, the aim being to identify the opportunities, to provide the scientific basis upon which practical applications can be developed, and to disseminate the research results and potential benefits. (Ellis and Goodall, 1999)

Most of the developments which exist at present are "fITst generation" systems, what might be called "fust having limited functionality with restricted control laws, usually applied locally to the actuators. It is however possible to envisage subsequent generations for which the mechatronic opportunities (and challenges) will be much greater:

6. REFERENCES A second generation of systems with a higher degree of functionality (e.g. lateral and vertical), integrated measurement systems for higher reliability and faulttolerance, and control laws which take a more complete view of the vehicle or train system.

AlIen, device, hold-offdevice, Allen, D.H. (1994). Active bumpstop hold-off Proc. Railtech 94, IMechEPaper C478/51013, C478/5/013, Birmingham UK. Binningham Anderson E, Bahr H V and Nilstam N G (1995) Allowing higher speeds on existing tracks ofthe the X2000 train trainfor for design considerations of Swedish State Railways Proc IMechE Pt F, Vol 209, No 2, pp 93-104 Beck, H.-P. and B. Engel (1996). Traction drive control with PI state controller and Kalman filter - first experimental results, Proc. 13th IFAC Congress, Vol P, pp 343 - 8, San Francisco. RV, Dukkipati, R V, Narayanaswamy, Sand Osman, M 0 M, (1993) Independently rotating wheel systems for railways - a state-of-the-art review Vehicle System Dynamics, Vo121, ,pp 297-330. Ellis Band Goodall R M (1999) The Mechatronic Train : Requirements and Concepts, Proceedings Train: of World Congress on Railway Research, Tokyo, Japan, Oct 1999. Ford R (1998) Tilting trains hold the key to Virgin's ambitious franchise Railway Gazette 1998. International, pp 707-710, Oct 1998. Frederich F (1999) Nullebenen-Konzept der 1237/8, Spurfuhrung, ZEV + DET Glas. Ann. 123 7/8, pp 269-278. Gilchrist A 0 (1998) The long road to the solution of the railway hunting and curving problems, Proc IMechE Part F, Vo1212 Vol212 pp219-226. Goodall R M (1997) Active railway suspensions: Implementation status and technological trends, Vehicle System Dynamics, Vol Vo128, 28, pp 87-117 Goodall R M and Li H (2000) Solid axle and independently-rotating railway wheelsets - a control engineering assessment ofstability, ofstability, Vehicle System Dynamics, Vo133, pp 57-67.

A third generation of systems offering a further increase in functionality, probably highly integrated with other vehicle dynamic systems such as traction and braking, and using information from facilities such as track profile databases, satellite positioning system etc.

5. CONCLUSIONS

Everywhere one looks at the technology of transport vehicles, the increasing importance of mechatronic solutions to vehicle design is apparent. Aircraft have essentially already made the transition, with full integration of controls and computation and the mechanical/aerodynamic mechanicaVaerodynamic structure through fly-bywire. The automotive industry has also made substantial changes with electronic engine controls and anti-lock braking systems, and other changes are imminent: brake-by-wire, active stability control, and steer-by-wire. steer-by-wire. The railway industry is following, but given the significantly longer product life cycle it is probably inevitable that it is lagging behind the aircraft and automotive industries. Nevertheless active tilting is now well established, and a number of practical developments have been reported recently which involve a high degree of control applied to the running gear. gear. It's interesting to see that, although the concepts of active secondary suspensions have been understood for many years, the actual take-up on real, operational vehicles is relatively low. They only really affect ride quality, for which the commercial return is relatively low, whereas the burgeoning interest in various forms of active steering is 31 31

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