Rickard Persson

Carbody tilting – technologies and benefits

Carbody tilting is today a mature and inexpensive technology allowing higher speeds in curves and thus reduced travel time. The technology is accepted by many train operators. Today more than 5000 tilting vehicles, defined as tilting carbodies, have been produced world-wide by different suppliers. Tilting trains can be divided into naturally tilted trains and actively tilted trains. However, also natural tilting will often include actuation to ensure satisfactory dynamic performance. The mechanical solutions for tilting involving pendulums or rollers are well proven. They have also become compact enough to avoid passenger area intrusion. The proportion of the lateral acceleration compensated by tilt has decreased over the years. In the early days of tilting train development, it was often assumed that the compensation should be 100%. Compensation of 50–70% are typically used in todays actively tilting trains, while natural tilting ones still retain compensation close to 100%. Recent developments in control have provided the market with more clever systems erasing the former problem with time delays. The result is beneficial for both ride comfort and low risk of motion sickness. As an example, running time simulations on the Swedish mainline Stockholm–Gothenburg have shown that the running time benefit for a tilting train vs. a non-tilting train, both with a top speed 275 km/h, is about 10%.

Active lateral secondary suspension with H ∞ control to improve ride comfort: simulations on a full-scale model

In this study, a full-scale rail vehicle model is used to investigate how lateral ride comfort is influenced by implementing the H ∞ and sky-hook damping control strategies. Simulations show that significant ride comfort improvements can be achieved on straight track with both control strategies compared with a passive system. In curves, it is beneficial to add a carbody centring Hold-Off Device (HOD) to reduce large spring deflections and hence to minimise the risk of bumpstop contact. In curve transitions, the relative lateral displacement between carbody and bogie is reduced by the concept of H ∞ control in combination with the HOD. However, the corresponding concept with sky-hook damping degrades the effect of the carbody centring function. Moreover, it is shown that lateral and yaw mode separation is a way to further improve the performance of the studied control strategies.

On Enhanced Tilt Strategies For Tilting Trains

This paper describes how many railways have taken tilting trains into operation on lines with horizontal curves with small radii. Tilting trains have vehicle bodies that can roll inwards, thereby reducing the lateral acceleration perceived by the passengers. Hence, tilting trains can run through curves at an enhanced speed. However, too much tilt can cause motion sickness among sensitive passengers. The tilt motions, generated by computer-controlled actuators should be optimized with care, taking the local track geometry and actual train speed into account. The paper presents tilt algorithms aimed at balancing conflicting objectives. Furthermore, the paper discusses the usefulness of route files (track geometry data) onboard the train and possible positioning systems.

Wear/comfort Pareto optimisation of bogie suspension

ABSTRACT Pareto optimisation of bogie suspension components is considered for a 50 degrees of freedom railway vehicle model to reduce wheel/rail contact wear and improve passenger ride comfort. Several operational scenarios including tracks with different curve radii ranging from very small radii up to straight tracks are considered for the analysis. In each case, the maximum admissible speed is applied to the vehicle. Design parameters are categorised into two levels and the wear/comfort Pareto optimisation is accordingly accomplished in a multistep manner to improve the computational efficiency. The genetic algorithm (GA) is employed to perform the multi-objective optimisation. Two suspension system configurations are considered, a symmetric and an asymmetric in which the primary or secondary suspension elements on the right- and left-hand sides of the vehicle are not the same. It is shown that the vehicle performance on curves can be significantly improved using the asymmetric suspension configuration. The Pareto-optimised values of the design parameters achieved here guarantee wear reduction and comfort improvement for railway vehicles and can also be utilised in developing the reference vehicle models for design of bogie active suspension systems.

Tilting Trains: Benefits and Motion Sickness

Carbody tilting is today a mature and inexpensive technology that allows higher speeds on curves, thus shortening travel time. The technology has been accepted by many train operators, but some issues are still holding back the full potential of tilting trains. This paper focuses on improving the benefits and limiting the drawbacks of tilting trains. This is done by quantifying the possible running time benefits compared with todays tilting trains, identifying what motion components have an influence on motion sickness, and finally quantifying the influence from the increased speed on these motion components. A running time analysis was made to show what potential there is to further improve running times by optimizing tracks and trains. Relations between cant deficiency, top speed, tractive performance, and running times are shown for a tilting train. About 9 per cent running time may be gained on the Stockholm-Gothenburg (457 km) main line in Sweden if cant deficiency, top speed, and tractive performance are improved compared with existing tilting trains. Introduction of non-tilting high-speed trains is not an option on this line due to the large number of 1000 m curves. However, tilting trains run a greater risk of causing motion sickness than non-tilting trains. Roll velocity and vertical acceleration are the two motion components that show the largest increase, but the amplitudes are lower than those used in laboratory tests that caused motion sickness. Higher curve speeds will increase carbody motions still further, but there are some possibilities to trade between vertical and lateral carbody acceleration by increasing or decreasing roll.

On-track tests of active vertical suspension on a passenger train

The classic way to design the suspension of a rail vehicle is to use passive elements such as dampers and springs; however, as sensors and actuators are getting more affordable and reliable, their potential benefit in the suspension system is increasingly studied. Active suspension can be used to keep ride comfort at an acceptable level or even improve it, while allowing tougher operation conditions or usage of lighter carbodies. Tougher conditions could be interpreted as higher speed or lower track quality, and lighter carbody means higher level of elastic vibrations. This paper is focused on the implementation and tests of active vertical suspension on the secondary suspension of a high-speed passenger electric multiple unit using hydraulic actuators and the skyhook method as the controller. Results from on-track tests indicate large ride comfort improvements.

Improving crosswind stability of fast rail vehicles using active secondary suspension

Rail vehicles are today increasingly equipped with active suspension systems for ride comfort purposes. In this paper, it is studied whether these often powerful systems also can be used to improve crosswind stability. A fast rail vehicle equipped with active secondary suspension for ride comfort purposes is exposed to crosswind loads during curve negotiation. For high crosswind loads, the active secondary suspension is used to reduce the impact of crosswind on the vehicle. The control input is taken from the primary vertical suspension deflection. Three different control cases are studied and compared with the only comfort-oriented active secondary suspension and a passive secondary suspension. The application of active secondary suspension resulted in significantly improved crosswind stability.

Strategies for Less Motion Sickness on Tilting Trains

Carbody tilting is today a mature and inexpensive technology that permits higher speeds in horizontal curves, thus shortening travel time. However, tilting trains run a greater risk of causing moti ...

Proposal for systematic studies of active suspension failures in rail vehicles

Application of active suspensions in high-speed passenger trains is gradually getting more and more common. Active suspensions are primarily aimed at improving ride comfort, wear or stability. Failure of these systems may not only just deteriorate the performance but it may also put vehicle safety at risk. There are not many studies that explain how a thorough study proving safety of active suspension should be performed. Therefore, initiating this type of study is necessary for not only preventing incidences but also for assuring acceptance of active suspension by rail vehicle operators and authorities. This study proposes a flowchart for systematic studies of active suspension failures in rail vehicles. The flowchart steps are solidified by using failure mode and effects analysis and fault tree analysis techniques and also acceptance criteria from the EN14363 standard. Furthermore, six failure modes are introduced which are very general and their use can be extended to other studies of active suspension failure. In the last section of the paper, the proposed flowchart is put into practice through four failure examples of active vertical suspension.

On-Track Tests and Simulation of Active Secondary Suspension on a Rail Vehicle : Research, Development and Maintenance

Ride comfort is one of the important criteria when designing and approving a new train. This parameter is negatively affected by low track quality or by increased train speed. One way to improve ride comfort in such operation conditions is to use active suspension control. However, the solution needs to be economic and reliable to remain attractive to industry. In this paper such an active suspension is developed and tested in a collaboration between KTH and Bombardier. The active control is implemented by replacing secondary vertical and lateral dampers with actuators. Skyhook control theory is used in combination with mode separation to calculate the reference force to the actuators. A two carbody train set manufactured by Bombardier is used as a test train. One of the cars has conventional passive suspension and is used as a reference car and the other is equipped with active secondary lateral and vertical suspension. Before carrying out the measurements, different failure scenarios of the active suspension were defined and studied in the multi-body simulation software Simpack. Active secondary vertical and lateral suspensions were finally tested together for the first time in Sweden in May 2013. Measurements were performed at different speeds up to 200 km/h on tracks around Stockholm. The results show a significant reduction of the vibration level in the carbody. According to the comfort values, up to 44% improvement is achieved.