Control of an active air bearing: simulation and experimental tests

The thesis final objective is the analysis and development of an active air bearing, based on a proportional pneumatic valve, able to regulate and maintain constant its lift from the basement. The aim is to carry on the previous thesis works by focusing on the analysis of the behaviour of the different components of the bearing and the study of the bearing itself in various working conditions, the comparison between two possible test-benches used, the deployment of a model suitable for the simulation of the device performances and the development of an active control system to make it behave as desired. The thesis is divided into three main parts. The first is dedicated to the analysis of the behaviour of the single components and the complete bearing, in order to understand their characteristics. The back pressure sensor and the valve characteristic are analysed evaluating the air film height as a function of the sensor voltage signal or as a function of the valve voltage input, respectively. Moreover, the tilting of the bearing is investigated during the lifting phase, trying to reduce this effect as much as possible. It is then evaluated the static characteristic curve of the system, obtaining the weight loaded on the bearing as a function of the lift level reached, for different valve openings. Then, it is performed the comparison between the Step test-bench used in this thesis and the Mager test-bench used in the previous. The second part focuses on the determination of an open loop model of the entire system. Different subsystems, representing the components, are deployed and studied and then, assembled to generate the model of the entire bearing. After that, some simulations are performed and compared with the experimental results. The third part is about the deployment of a control action. The PID control is adopted: two versions are tested and the parameter are tuned in order to optimize the control action. Different laboratory tests of the closed loop control are performed, to check its behaviour. From the first part of the thesis, different graphs are obtained, fundamental for generating the model and develop the control action. Some of the graphs are useful for the evaluation of the behaviour of the components, some other are preparatory for the generation of the control action. For what concerns the second part, the model relative to the proportional valve reflects well the real behaviour of the component, with a good overlapping of the curve representing the airflow of the valve against the input current signal. Instead, the simulations coming from the complete open loop model don’t comply with the laboratory results, even after some tuning of the parameters. The part relative to the control action returned good results in terms of following the air film height required. The tuning performed allowed to obtain improvements in the control action. In the end, good results have been reached in the deployment of the control action, even though it is only a first implementation of the action, thus further improvements are still required. Instead, the obtained model returns results which are only partially reflecting the real behaviour of the system, therefore, it is necessary to revise it to make it better comply the real tests.