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.