46 research outputs found
Operational reliability calculations for critical systems
Reliability theory deals with the effect of mean time to repair upon overall system failure rates, but for critical systems such calculations are not what is required because an important performance criterion relates to operational failures, which are fundamentally different to unsafe failures: essentially they are the result of the system-level response to avoid unsafe failures. This paper introduces the particular problem for critical systems in general, presents an analysis of some of the relevant conditions and provides some simulation results in the context of a railway active suspension application that illustrate the overall effects and trends
Hydraulic actuation technology for full- and semi-active railway suspensions
The paper describes a simulation study that provides a comprehensive comparison between full-active and semi-active suspensions for improving the vertical ride quality of railway vehicles. It includes an assessment of the ride quality benefits that can theoretically be achieved with idealised devices, and also examines the impact of real devices based upon hydraulic actuation technology
Wheel-rail profile condition monitoring
Increased railway patronage worldwide is putting pressure on rolling stock and infrastructure to operate at higher capacity and with improved punctuality. Condition monitoring is seen as a contributing factor in enabling this and is highlighted here in the context of rolling stock being procured with high capacity data buses, multiple sensors and centralised control. This therefore leaves scope for advanced computational diagnostic concepts. The rail vehicle bogie and associated wheelsets are one of the largest and most costly areas of maintenance on rolling stock and presented here is a potential method for real time estimation of wheel-rail contact wear to move this currently scheduled based assessment to condition based assessment.
This technique utilises recursive ‘grey box’ least squares system identification, used in a piecewise linear manner, to capture the strongly discontinuous nonlinear nature of the wheel-rail geometry
Contact force estimation in the railway vehicle wheel-rail interface
Increased patronage of railways in the UK in the past 20 years has put demands
on rolling stock to operate at peak availability with reduced time available for maintenance.
One possible tool to enable this is the use of real time fault detection and diagnosis on board
railway vehicles to detect faulty components and provide information about the current running
condition of the system. This paper discusses the development of one such technique for the
estimation of creep forces of the wheel-rail contact. Real time knowledge of which could be used
to predict wear of the wheel tread and rail head, predict the formation of rolling contact fatigue,
and identify any areas of low adhesion present on the network. The paper covers development
of a full vehicle nonlinear contact mechanics model, development of the Kalman-Bucy filter
estimation technique and how the technique will be developed and validated in the future
Reliability of 2-out-of-N:G systems with NHPP failure flows and fixed repair times
It is commonplace to replicate critical components in order to increase system lifetimes
and reduce failure rates. The case of a general N-plexed system, whose failures are
modeled as N identical, independent nonhomogeneous Poisson process (NHPP) flows,
each with rocof (rate of occurrence of failure) equal to λ(t), is considered here. Such
situations may arise if either there is a time-dependent factor accelerating failures or
if minimal repair maintenance is appropriate. We further assume that system logic for
the redundant block is 2-out-of-N:G. Reliability measures are obtained as functions of
Ï„ which represents a fixed time after which Maintenance Teams must have replaced any
failed component. Such measures are determined for small λ(t)τ, which is the parameter
range of most interest. The triplex version, which often occurs in practice, is treated in
some detail where the system reliability is determined from the solution of a first order
differential-delay equation (DDE). This is solved exactly in the case of constant λ(t),
but must be solved numerically in general. A general means of numerical solution for the
triplex system is given, and an example case is solved for a rocof resembling a bathtub
curve
The ALSTOM benchmark challenge on gasifier control
Integrated gasification combined cycle power plants are being developed around the world to provide environmentally clean and efficient power generation from coal. As part of the UK’s Clean Coal Power Generation Group, ALSTOM (formerly GEC ALSTHOM ) has undertaken a detailed feasibility study on the development of a small-scale prototype integrated plant (PIP), based on the air-blown gasification cycle. In pursuit of this goal the ALSTOM Power Technology Centre (formerly the GEC ALSTHOM Mechanical Engineering Centre) has produced a comprehensive dynamic model
and control philosophy for the PIP. The gasifier is one component of the model which, being a highly
coupled multi-variable system with five inputs (coal, limestone, air, steam and char extraction) and
four outputs (pressure, temperature, bed mass and gas quality), has been found to be particularly
difficult to control. For this reason the gasifier, together with its associated control specification,
operating constraints and various disturbance characteristics, has been selected as the subject for this control challenge. This paper provides a brief background to the problem and describes the control
specification and closed-loop tests to be performed
Condition monitoring of rail vehicle bogies
This paper details a range of work carried out by the authors within the general theme of advanced condition monitoring possibilities for rail vehicle bogies. Maintenance of rail vehicle bogies represents one of the largest areas of whole vehicle running costs and their efficient
operation is of safety critical importance to the entire rail system. Currently the majority of maintenance is carried out on a scheduled basis which can be time consuming, costly and potentially not effective at fault detection. This paper reviews concepts that could allow real time detection of the condition of the bogie so as to reduce vehicle out of service time and improve safe operation. Concepts reviewed are: the use of condition monitoring for detection of
suspension component condition; detection of low adhesion conditions; and assessment of the wheel-rail interface condition
The benefits of mechatronically-guided railway vehicles: A multi-body physics simulation study
Mechatronically-guided railway vehicles are of paramount importance in addressing the increasing interest in reducing wheel-rail wear and improving guidance and steering. Conventional passively-guided rail vehicles are limited by the mechanical constraints of the suspension elements. Currently, a typical rail vehicle suspension needs to be sufficiently stiff to stabilize the wheelsets while being complaint enough to negotiate curved track profiles. The suspension is therefore a compromise for the contradictory requirements of curving and stability. In mechatronic vehicles, actuators are used with the conventional suspension components to provide additional stiffness or damping forces needed to optimise a vehicle for a wide variety of scenarios, and not rely on a sub optimal combination of passive components. This research demonstrates the benefits of active guidance and steering when compared to a conventional vehicle using simulation results from a multi-body simulation software Simpack. It also provides insights into the relative performance of the mechatronic schemes. The Simpack modeling allows for a complex model with high fidelity, which provides an additional level of proof of the control algorithms working on a real rail vehicle. Each vehicle is assessed in terms of guidance on straight track, steering on curved track, actuation requirements and wheel-rail wear. Significant benefits are demonstrated in one of the guided vehicles with independently-rotating wheelsets
LQG control of a high redundancy actuator
A high redundancy actuator, comprising a relatively
large number of actuation elements, is being developed for
safety critical applications. Some classical control results have
previously been reported and this paper will focus on evaluation
of the LQG control design. Three different design approaches
will be presented and compared under different types of typical
faults in the sub-actuation elements. Overall a LQG design
using a physically motivated reduced order model appears to be the best approach
Improving the reliability and availability of railway track switching by analysing historical failure data and introducing functionally redundant subsystems
Track switches are safety critical assets that not only provide flexibility to rail networks but also present single points of failure. Switch failures within dense-traffic passenger rail systems cause a disproportionate level of delay. Subsystem redundancy is one of a number of approaches, which can be used to ensure an appropriate safety integrity and/or
operational reliability level, successfully adopted by, for example, the aeronautical and nuclear industries. This paper models the adoption of a functional redundancy approach to the functional subsystems of traditional railway track switching arrangements in order to evaluate the potential increase in the reliability and availability of switches. The
paper makes three main contributions. First, 2P-Weibull failure distributions for each functional subsystem of each common category of points operating equipment are established using a timeline and iterative maximum likelihood estimation approach, based on almost 40,000 sampled failure events over 74,800 years of continuous operation. Second,
these results are used as baselines in a reliability block diagram approach to model engineering fault tolerance, through
subsystem redundancy, into existing switching systems. Third, the reliability block diagrams are used with a Monte-Carlo simulation approach in order to model the availability of redundantly engineered track switches over expected asset lifetimes. Results show a significant improvement in the reliability and availability of switches; unscheduled downtime
reduces by an order of magnitude across all powered switch types, whilst significant increases in the whole-system reliability are demonstrated. Hence, switch designs utilising a functional redundancy approach are well worth further investigation. However, it is also established that as equipment failures are engineered out, switch reliability/availability
can be seen to plateau as the dominant contributor to unreliability becomes human error