Over the last decades electronics and controls have invaded virtually all technological fields. In railways, traction control has evolved from being purely mechanical to modern power electronics and software based systems.

However, suspension technology of railway vehicles has not experienced a comparable integration of electronics. In recent years there have been growing efforts from mechatronics to study active suspension technologies based on sensors, controllers and actuators. Mechatronics is the science of "technical systems operating mechanically with respect to at least some kernel functions but with more or less electronics supporting the mechanical parts decisively" (University of Linz, Austria).

The basic principle of such a mechatronic integration of electronic control into mechanical suspensions is illustrated by Figure 1. Whereas passive suspensions based on springs and bumpers only react passively to forces coming from wheel-track-interaction, active suspension technology measures these forces by means of sensors, calculates an appropriate reaction by means of controllers and realises this reaction on the mechanical system by means of actuators.

The main goal of mechatronic solutions for running gear is improved riding comfort and reduction of weight, wear and complexity.

Figure 1: Scheme of an active suspension

Fields of application

A modern railway vehicle consists of seven dynamic masses: the car-body, two bogies and four wheel-sets. The wheel-sets are connected to the bogies via primary suspensions, bogies are attached to the car-body via secondary suspensions (Figure 2). Whereas first applications of active secondary suspensions already exist, active solutions for primary suspensions are much further down the road.

Figure 2: Simplified scheme of railway vehicle

Source: IZT

Secondary suspensions

Tilting is a specific form of active secondary suspension and is generally accepted and in wide-spread use all over the world. The concept of active secondary suspensions can be generalised to other applications. The damping of the car-body against wheel-track forces can be optimised by an intelligent sensor-actuator-system (cf. Figure 3).

Figure 3: Active secondary suspension control scheme

Source: Goodall, Kortüm 2000. 

Primary suspensions

The active steering of wheels and wheel-sets is a much more difficult step than active secondary suspensions. Developments in this field range from electronically controlled single-axle running gear to wheelsets with two independently rotating wheels instead of a common axle and directly-steered wheelpairs (Figure 4).

Figure 4: Directly-steered wheels

Source: Goodall, Kortüm 2000. 

The following evaluation focusses on mechatronic developments for primary suspensions since they are more relevant from an energy efficiency perspective.

In the field of secondary suspensions, besides wide-spread use of tilting there have been numerous attempts and research efforts. ADtranz in Sweden has looked into "semi-active" electronically-controlled lateral dampers for the X2000 tilting train. Similar efforts have been done at ABB, Alstom and others. In Japan hydraulic and pneumatic actuators have been tested on the WIN350 train.

Implementation of active primary suspensions is still further away. Some research has been conducted in the Netherlands for the Rotterdam tram as well as in Germany and Austria. The Copenhagen S-trains with (LINK!) KERFs do not use active suspensions in the strict sense used here, but represent an important development in this direction.

An international research consortium "Mechatronic technologies for trains of the future" (funded by the European Commission) is currently studying the potential of these and other mechatronic developments for future train design.

• Weight reduction
• Reduced mechanical complexity

Reduction of wear and tear

Less wear on wheels and track through improved curving capability

Weight reduction

Active steering makes two-axle bogieless vehicles feasible, offering advantages such as

• reduced mechanical complexity (which of course is paid for by a higher degree of electronic control complexity)
• reduced vehicle weight
• reduced traction/braking requirements
• reduced energy consumption
• reduced track damage

Extra functionalities

A mechatronic approach may bring added value of a different kind, e.g. the integration of the suspension, guidance, drive and braking sub-systems which today are designed and controlled separately. The use of differential torque control to achieve active steering or guidance is a step towards higher degrees of system integration. In addition, condition and fault diagnosis for maintenance purposes could be provided at low extra cost, making use of the sensors fitted for mechatronic control.

Technological inertia

Primary suspensions are an extremely fundamental and safety-critical sub-system of a railway vehicle. Therefore it will not be easy to replace the well-known and mature bogie technology by innovative technologies.

Safety and reliability

Primary suspensions are still confronted with a number of safety problems. More research is needed.

Actively controlled running gear meets very high acceptance barriers due to doubts about safety and reliability. Success factors are:

Thorough risk assessment

Make a thorough risk assessment of the whole vehicle rather than running gear alone.

Communication strategy

In order to overcome scepticism, a convincing communication strategy is essential. The focus should be put on successful use of active controlled mechanical elements in other industries:

• automotive: Anti-lock braking system (ABS), Active stability control (ASC)
• aerospace: fly-by-wire etc.

But also in railways, tilting trains are an example of successful introduction of active control elements and are seen by experts as the "tip of an iceberg" (Goodall, Kortüm 2000) that could facilitate introduction of mechatronic solutions in other fields.

It is, at the present stage of development, impossible to assess the energy saving potential of active controlled running gear.

The "Mechatronic technologies for trains of the future" project (mentioned in "state of development") is guided by the objectives (among others):

• train mass per unit payload reduced by 40 % (by eliminating bogies)
• energy consumption reduced by 30 % (consequence of mass reduction)

This seems very ambitious, but not altogether unrealistic in a very long-term perspective.

The additional power need caused by control equipment for active steering is low and of the order of tens of Watts per axle (Ellis, Le et al. 1999). So this does not put in danger the net reduction effect.

Reduced noise emission

The "Mechatronic technologies for trains of the future" project (cf. "state of development") is guided by the objective (among others):

• noise emission at medium/high frequencies reduced by 10 dB(A) (due to fewer axles, lower axle load, better curving)

• Reduced energy costs
• Reduced maintenance: reduced wear of wheels and rails / reduced complexity (Reduced maintenance could especially be an issue for low speed systems (trams, metros, light rail) having tighter curves)