Active fault tolerant control applied to REPOINT, a novel railway track switch

Railway networks are fitted with switches and crossings that enable trains to move from one track to another however they present a single point of failure. Existing track switches actuation is performed in the open loop presenting a research gap where closed-loop fault tolerant control can be applied to track switch actuation in order to improve railway network performance. A new railway track switch, REPOINT has been developed at Loughborough University with a new electromechanical design that incorporates actuator redundancy to improve the reliability of track switch operation. This paper looks at the development and validation of a sensor fault detection, identfication and accomodation scheme applied to a detailed non-linear model representing the laboratory scale demonstrator of the REPOINT concept. A residual-based fault diagnosis scheme is developed from the comparison of estimates generated by a bank of observers and output measurements. In the presence of sensor faults, a reconstructed signal from the fault detection algorithm is used to replace the measured signal for feedback control and thus safe switching position control is achieved. The results demonstrate that using a reliable fault tolerant control configuration could increase the availability and reliability of the REPOINT track switch

Benefits of mechatronically guided vehicles on railway track switches

Conventional rail vehicles struggle to optimally satisfy the different suspension requirements for various track profiles, such as on a straight track with stochastic irregularities, curved track or switches and crossings (S&C), whereas mechatronically-guided railway vehicles promise a large advantage over conventional vehicles in terms of reduced wheel-rail wear, improved guidance and opening new possibilities in vehicle architecture. Previous research in this area has looked into guidance and steering using MBS models of mechatronic rail vehicles of three different mechanical configurations - secondary yaw control (SYC), actuated solid-axle wheelset (ASW) and driven independently-rotating wheelsets (DIRW). The DIRW vehicle showed the best performance in terms of reduced wear and minimal flange contact and is therefore chosen in this paper for studying the behaviour of mechatronically-guided rail vehicles on conventional S&Cs. In the work presented here, a mechatronic vehicle with the DIRW configuration is run on moderate and high speed track switches. The longer term motivation is to perform the switching function from on-board the vehicle as opposed to from the track as is done conventionally. As a first step towards this, the mechatronic vehicle model is compared against a conventional rail vehicle model on two track scenarios - a moderate speed C type switch and a high speed H switch. A multi-body simulation software is used to produce a high fidelity model of an active rail vehicle with independentlyrotating wheelsets (IRWs) where each wheel has an integrated ’wheelmotor’. This work demonstrates the theory that mechatronic rail vehicles could be used on conventional S&Cs. The results show that the mechatronic vehicle gives a significant reduction in wear, reduced flange contact and improved ride quality on the through-routes of both moderate and high speed switches. On the diverging routes, the controller can be tuned to achieve minimal flange contact and improved ride quality at the expense of higher creep forces and wear

Modelling and control of a high redundancy actuator

The high redundancy actuation concept is a completely new approach to fault tolerance, and it is important to appreciate that it provides a transformation of the characteristics of actuators so that the actuation performance (capability) degrades slowly rather than suddenly failing, even though individual elements themselves fail. This paper aims to demonstrate the viability of the concept by showing that a highly redundant actuator, comprising a relatively large number of actuation elements, can be controlled in such a way that faults in individual elements are inherently accommodated, although some degradation in overall performance will inevitably be found. The paper introduces the notion of fault tolerant systems and the highly redundant actuator concept. Then a model for a two by two configuration with electromechanical actuation elements is derived. Two classical control approaches are then considered based on frequency domain techniques. Finally simulation results under a number of faults show the viability of the approach for fault accommodation without reconfiguration

Requirements analysis for high redundancy actuation

This document introduces the idea of high redundancy actuation. Typical requirements for actuators in different applications are discussed, and a synthesis of the most important parameters is presented. To be successful, a high redundancy actuator needs to satisfy the same kind of requirements. Based on these, tentative parameters for an experimental verification of the high redundancy concept are proposed

Robust control of a high redundancy actuator

The High Redundancy Actuator project deals with the construction of an actuator using many redundant actuation elements. Whilst this promises a high degree of fault tolerance, the high number of components poses a unique challenge from a control perspective. This paper shows how a simple robust controller can be used to control the system both in nominal state and after faults. To simplify the design task, the parameters of the system are tuned so that a number of internal states are decoupled from the input signal. If the decoupling is not exact, there may be small deviation from the nominal transfer function, especially when a fault has occurred. The robustness analysis ensures that the system performs well for all expected behaviour variations

Increasing reliability by means of efficient configurations for high redundancy actuators

A high redundancy actuator (HRA) is composed of a high number of actuation elements, increasing both the travel and the force above the capability of an individual element. This provides inherent fault tolerance: if one of the elements fails, the capabilities of the actuator may be reduced, but it does not become dysfunctional. This paper analyses the likelihood of reductions in capabilities. The actuator is considered as a multi-state system, and the approach for k-out-of-n:G systems can be extended to cover the case of the HRA. The result is a probability distribution that quantifies the capability of the HRA. By comparing the distribution for different configurations, it is possible to identify the optimal configuration of an HRA for a given situation

Model-based controller design for a lift-and-drop railway track switch actuator

Track switches are essential in order to enable railway vehicles to change routes however they are also the largest single cause of failure on the railway network. A new generation of switching concepts are emerging from projects like In2Rail, REPOINT and S-Code that promise to improve rail network performance through the use of new mechanisms, monitoring and control systems. This paper focusses on modelling and control of a lab-demonstrator from the REPOINT project. Unlike conventional track switch machines, this actuator needs closed loop feedback control. First, a detailed simulation model of the actuator is developed and validated against experimental results. Two model-based control designs are then developed and tested: a classical cascaded P/PI controller and a modern state feedback controller. The two controllers are compared and it is found that, whilst there are some performance differences, both meet the requirements for use in a redundantly actuated REPOINT switch

Adaptive control of a High Redundancy Actuator using the geometric approach

The High Redundancy Actuator project deals with the construction of an actuator using many redundant actuation elements. Whilst this promises a high degree of fault tolerance, the high number of components poses a unique challenge from a control perspective, especially when actuation elements are used in series.This paper describes how an adaptive control scheme can be used to deal with faults in a High Redundancy Actuator. This is based on previous results leading to a simplified model of the HRA with serial elements. In case of the fault, the parameters change, but the otherwise the deviation from the simplified model is minimal.This approach has two benefits. For one, it can restore the original system dynamics even after a fault has occurred. The parameter estimate can also be used for health monitoring purposes, because it reflects the number of effective faults in the HRA.</p

LQR control applied to a novel track switch actuator

Existing railway track switches use open-loop actuation to enable trains to take different routes. A new type of railway track switch called REPOINT with a different electromechanical design to conventional track switches has been developed at Loughborough University that requires closed-loop control in order to perform its track switching function. This abstract proposes the use of a Linear Quadratic Regulator (LQR) controller to achieve track switch position control applied to the REPOINT switch. The results showed that the LQR controller enables successful switching of this novel railway track switch and satisfies the control requirements of a REPOINT track switch

High redundancy in actuation

Actuation, the controlled movement and positioning of objects, is an essential function of many technical systems. It is crucial in many applications from central heating to aircrafts, and without actuation, the function or even the safety of the system would suffers. For example an aircraft is steered using control surfaces, and if the actuation of these surfaces fails, the aircraft may crash. Therefore, actuation is often provided by using several (typically between 2 and 4) redundant actuation elements. If one element fails, another takes over, and harm can be avoided. While this solution works, it involves increased cost, weight and energy use, reducing the efficiency of the system considerable. This project on high redundancy actuation investigates the use of a high number of actuation elements, such as 10 or even 100. This is a bionic (or bio-mimetic) idea: the use of actuation elements is similar to the composition of a muscle from many individual muscle fibres. Just like the muscle is highly resilient to damage in individual fibres (causing sore muscles, but no loss of movement), high redundancy actuation is highly reliable even if several elements have failed. The reliability analysis shows that this approach provides the same level or even superior protection against faults, without the loss of efficiency involved in the traditional solution. The basic advantage is that the law of large numbers applies, which provides a much more accurate prediction of how faults will affect the actuation elements over time. In the aircraft example, this would provide lighter actuators that provide superior reliability, leading to better fuel economy and easier maintenance. The main scientific problem of this project is how to deal with the complexity of using a high number of elements together. The results show that it is possible to determine the reliability of the system, and it is also possible to control the many elements as if they are just one big actuator. The next phase of the project is dealing with the technological challenges of combining many actuation elements. A simple experiment with four elements has been completed, and a demonstrator with 16 elements is being built. These experiments are used to demonstrate the resilience to faults, and understand the practical control issues at hand. A project leading to a more advanced version with up to 100 elements is currently being prepared. While the developed theories can be extended easily to consider such configurations, the practical difficulties of designing and manufacturing such a solution are challenging. The goal is to demonstrate that high redundancy actuation is feasible with the currently available technology, and to get an idea of the manufacturing issues involved