A New Approach to Vibration Mitigation of Moving-load Problems

Dynamic problems with moving loads are analytically and experimentally more difficult to solve than problems with conventional stationary (but dynamic) loads. They are however very commonplace, occurring in traffic flows on bridges, roads and rail tracks, pipelines carrying fluids, machine parts such as clutches, brakes and sliders, machining operations involving metal removal, wood saws and cranes. It is believed that moving-load problems are likely to be extremely important in many modern machine applications, especially where low mass and very high speeds of operation are required, such as computer disk drives and high speed machining. Foot bridges (for example, the Millennium Bridge) and light-weight long-span floors are particularly susceptible to excitation by people walking (and also dancing or jumping for the latter) and have now become of particular concern to civil and structural engineers. The development of high speed trains in recent years has also posed serious and new technological challenges. Typical problems caused by moving loads include generation of unwanted noise, chatter, nonlinear and self-excited vibrations leading to instability and/or large-amplitude limit cycles. It must be recognised that it is inevitable that moving loads excite vibration. So the central research issue is not to completely suppress vibration excited by moving loads, because that is impossible to achieve, but rather to mitigate vibration thus caused. Vibrations excited by moving loads are not stationary in general and are characterised by a wide frequency range and high-amplitude, hence it is very difficult to reduce and control. The purpose of this new proposal is to study vibration mitigation of moving-load dynamic problems in a systematic way.The non-stationary nature of moving-load problems creates a two-fold difficulty: (1) time-varying frequencies form bands as the moving structure traverses the spatial domain and (2) the frequencies also vary with the speed of the moving structure. Therefore previously established methodologies, such as assigning frequencies to fixed values and instantaneous optimal control, would not work well or possibly not at all. New control concepts have to be explored, which is the major motivation of this new project.Structural vibration control may be based on several strategies: (1) classical linear optimal control, (2) pole and zero assignment, (3) instantaneous optimal control, (4) independent modal space control, and (5) bounded state control. While many works on structural vibration control are based on the first-order state-space formulation, direct treatment of the second-order equation of motion is the natural framework to vibration engineers. In addition, receptance-based inverse methods, first put forward for symmetric systems by Ram and Mottershead in 2007 and then extended to asymmetric systems by this applicant in 2009, have a particular appeal in that receptances are easy to measure and are required at only a small number of degrees-of-freedom; there is no need to know mass, stiffness or damping matrices so that numerical modelling errors can be avoided (a finite element model, though useful, is not required). Active vibration control has been a major research topic and has shown considerable promise in solving practical engineering applications in recent years, but with a relatively very small number of exceptions, active solutions of moving-load problems have rarely been studied. The abundant wealth of active control methods and theories used in non-moving-load problems have largely been left out of moving-load problems. This glaring absence presents a not-to-be-missed opportunity to create a new body of knowledge and apply it to novel applications. The applicant aims to bring fresh ideas into solving the long-standing but continuously expanding field of moving-load dynamic problems and achieve vibration mitigation in moving-load dynamics using active and passive control.

Systems with contacting components in relative motion present moving-load problems and are prone to large-amplitude vibration or noise. This project provides an excellent opportunity to study them in greater depth and more importantly to provide means of mitigating the vibration thus caused. Both the original theoretical advancement of scholarship and the potential for engineering applications will be profound. The research to be carried out will allow faster operation of machines and vehicles and produce savings in material and maintenance cost offering considerable economic benefit to many industries. Each of the four supporters of this project will be able to draw on the outcome. From the projects they have either been engaged in themselves or commissioned contractors to carry out, it is clear that they encounter many moving-load problems and are trying to resolve them. A few examples are listed below. Cracks on road surfaces are a common occurrence and cost a large amount of time and money to repair. The Highways Agency has commissioned several projects on road surface cracks in the last few years costing hundreds of thousands of pounds. One project is Non-Destructive Methods for Determining Crack-Depth (Reference number: Y104845) carried out by TRL (now L3-TRL Technology) during 09/2003 to 09/2005. Another project is Improved Design of Overlay Treatments (YY91885) also carried out by TRL during 01/2003 to 03/2006. The Highways Agency has also financially sponsored a number of projects on the structural integrity and development of materials of infrastructures. It is believed that moving loads from traffic play the most significant role in causing road surface cracks. Reports show that over the 30-year period from 1969 to 1999/2000 the number of broken rails on Railtrack (British Rail before 1994) has stayed almost constant at an average of 767 per year and a standard deviation of 128 but there has been a steady increase of the number of broken rails in more recent years, and similarly the number of defective rails removed per year has increased almost linearly from about 1,250 in 1969 to about 8,700 in 1999/2000, approximately a 600 percent increase. Broken and defective rails not only increase maintenance cost and outage time but also contribute to loss of life. Four passengers died when an InterCity express derailed at high speed near Hatfield on 17th October 2000. Subsequent investigations indicated the derailment was most likely caused by a rail that had broken from gauge corner crack defects. Moving loads are capable of exciting vibrations of more frequencies and higher amplitude and hence conspire to cause more broken and defective rails. Network Rail has been actively involved in their own research activities or sponsoring external projects. For example, they commissioned a comprehensive study of non-destructive evaluation of rails to the Transportation Technology Center, Inc. (TTCI), a subsidiary of the Association of American Railroads. TTCI provided a 114-page long report Rail Failure Assessment for the Office of the Rail Regulator in 2000. The Highways Agency and Network Rail, Ford Motor Company and Atkins (the latter two are also sponsors of the applicant's recently completed EPSRC project Moving Load Distributions in Structural Dynamics (EP/D057671/01), June 2006 - May 2009) wish to see fundamental research into moving-load dynamic problems studied and generic results applied. They look to university academics to produce creative ideas and provide such results (please see Network Rail's support letter). It is expected that the outcome of this project could influence their design methodology and maintenance strategy. The construction industry should benefit from the knowledge of moving-load effects and can improve design and construction. Road and railway users will also benefit from fewer outages and shorter journeys. The general public will enjoy a quieter environment.