Fault Tolerant Strategy for Actively Controlled Railway Wheelset

this paper studies the key issue of fault tolerance for actively controlled railway wheelsets. It assesses failure modes in such systems, with a focus on actuator failures, and consequence of those hardware failures. It seeks to establish the necessary basis for control reconfiguration to ensure system stability and performance in the event of a faulty, without the need for hardware redundancies. A number of control schemes (with and without faults) are included in the study. Both analytical and simulation results are presented. Keywords-Railway; Wheelset; Active control; Stability; Actuator; Fault tolerance. INTRODUCTION Conventional wheelset for the railway vehicle is composed of the two coned (or profiled) wheels rigidly fixed to a common axle to rotate at the same angular velocity. When an unconstraint wheelset rolling along the track it is displaced laterally due to track irregularities, the rolling radii therefore are different because of the profiles of these two wheelset. Consequently, different forward speeds obtained for each wheelset due to the difference rolling radii to provide a natural centering/curving action. However, an unconstrained wheelset also presents a problem of kinematic instability known as the “Kinematic Oscillation” or wheelset “hunting” [1,2]. Traditionally the wheelset is stabilized by using passive suspensions on conventional rail vehicle, but such additional stiffness affects the pure rolling action of the wheelset around the curve. It has been theoretically proven that to this design conflict between stability and curving performance can be solved by applying active control instead of conventional passive components within the primary suspension of railway vehicle [2]. Passive components in the primary suspensions can be designed in such a way not to fail in order to maintain the stability and steering performance of railway vehicle and they are generally accepted as “safe” in railway industry. However, any new technology must prove that it can cope with any failures to demonstrate that any component faults would not lead to the system failure such that passenger safety is not compromised under such conditions. From a practical point of view, any active control scheme must be also able to maintain an effective operation of a rail system in order to meet the necessary standard of reliability [3]. Hardware redundancy technique may be used in the system to guarantee safety operation of such a system. Whilst it may be acceptable to apply the above technique in sensors due to their relatively low cost, it is far more difficult to justify the use of multiple actuators in a cost effective manner for redundancy or accommodate those within the limited space of railway bogie [3]. There are two main approaches for fault tolerant control systems. The first philosophy relies on the existing system redundancies to achieve acceptable performance in the event of component failures. In this type of systems, once the controllers designed, it will remain stable. It should be noted that the redundancies in such a system are usually in hardware forms. The second methodology takes a completely different approach to achieve fault tolerance. It involves such procedures as real-time fault detection, isolation, and control system reconfiguration. The redundancy in such a system may be an analytical form [4] and help to minimize the use of the hardware redundancies in order to keep the overall cost down [5]. The object of this study is to develop the fault tolerance approaches without using redundant actuators to provide stability across a range of operation conditions with different failure modes. It investigates the possibilities/feasibilities of re-configuring the controller based on the use of remaining actuator(s) in the system. For this study, the paper will review first a number of different control methods for railway wheelsets in the normal condition to understand how control for stability and/or curving performance is achieved. A thorough assessment of failure modes and adverse effect of the