PhD Defence at DTU Mechanical Engineering

PhD Defence 9th May: "Combining Gas Bearing and Smart Material Technologies for Improving Machine Performance"

Monday 01 May 17
Bo Bjerregaard Nielsen from DTU Mechanical Engineering defends his PhD, "Combining Gas Bearing and Smart Material Technologies for Improving Machine Performance", Tuesday 9th May from 13:00 - 17:00. The defence takes place in Auditorium 074, Building 421 at The Technical University of Denmark. Professor Ilmar Santos from DTU Mechanical Engineering is supervisor, examiners are: Associate Professor Casper Schousboe Andreasen, DTU Mechanical Enginnering, Dr. Philip Bonello, The University of Manchester, United Kingdom and Professor Marcellog Svi, Federal University of Rio Janeiro, Brazil. Chairman: Associate Professor Niels Leergaard Pedersen, DTU Mechanical Engineering.

According to industry leaders, the world is on the verge of the fourth industrial revolution in which the Internet of Things and cyber-physical systems are central concepts. Where the previous industrial revolution evolved around electronics, IT and automated production on machine level, Industry 4.0 will enable a much stronger interaction between all of these technical achievements, from factory level all the way down to the individual machine elements. This can be exemplified by its the impact on machine maintenance. Nowadays, to avoid unwanted machine stops, maintenance cycles are scheduled based on the principle of the weakest link, e.g., the minimum expected lifetime of any machine element. In the future individual machine elements will not only send information about their performance, they will also be able to compensate for "wear and tear" or adapt to new operating conditions autonomously in coordination with adjacent machine elements. This requires mechatronic machine elements, which combine traditional passive mechanical components with sensors, actuators, electronics and computer algorithms, which thereby become "self-acting" machine elements, e.g. the piezoelectric air foil bearing (PAFB).

One way of supporting a rotor running at higher speed is by using air foil bearing (AFB). An AFB utilizes the aerodynamic pressure created by the relative velocity difference between the rotor and the bearing surface. In an AFB the bearing surface is flexible and is made up by a thin top foil and a bump foil placed between the top foil and bearing housing. The PAFB combines the traditional AFB with piezoelectric material incorporated into the top foil. This creates a link between the mechanical domain of the traditional machine element and the electrical domain, i.e., ultimately a computer. The thesis deals with the development of the PAFB, and gives three main contributions: the design of a multifunctional test facility; the development of a state-of-the-art mathematical model of the PAFB and AFB; and interpretation of numerical results contributing to the understanding of both AFBs' and PAFBs' static and dynamic behaviours. The facility is designed to experimentally study the PAFB and its sub-systems. This allows for validation of mathematical models and gain of further knowledge of the PAFB's static and dynamic behaviour. The mathematical models, based on the finite element method (FEM), are created as a combination of AFB models and models of piezoelectric material and their constitutive equations. The model includes journal, air film, piezoelectric top foil (PTF), bump foil and electrical circuit. It takes non-linear effects resulting from the aerodynamic pressure into account allowing for a separation of the top foil and bump foil. Numerical results obtained with a sub-model of the PTF shows good agreement with experiments, while simulations of a passive PAFB closely resembles results obtained with a non-linear AFB model known from literature.

A numerical investigation shows that rotor-bearing sub-harmonic vibrations associated with large journal unbalance can be eliminated when the top foil is only partly supported by the bump foil, i.e., "shallow pocket" effect. The aerodynamic forces are significantly affected by the deformations of the PTF caused by the piezoelectric material due to an electrical potential difference (EPD) imposed between the electrodes. It is possible to increase the aerodynamic forces, and thereby the bearing load capacity, by a factor of two. The future steps in the development of PAFB are the design of feedback control laws and the experimental validation of a fully-controlled PAFB aided by the designed test facility and mathematical model derived in the thesis.

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22 JANUARY 2018