A comparative study on the control of active magnetic bearings

Abstract

The aim of this project is to perform a comparative study on different control techniques applied to active magnetic bearings (AMBs). The project will involve the evaluation of two suitable nonlinear modem control techniques on an existing model to illustrate their superior performance over conventional linear control techniques. The techniques under investigation are classical PD control, fuzzy logic control and sliding mode control. As a first step the experimental model was characterised and found to be inadequate for a meaningful comparative control study. The physical model was improved in terms of sensor linearization, power amplifier configuration and magnetic circuit layout. A comprehensive matched simulation model of the experimental model is then developed to serve as the design platform for the mentioned controllers. The comparative study commences with the optimisation of a classical PD controller for the experimental model. This controller is used as benchmark for performance comparison. An equivalent linear fuzzy logic controller is then derived from the PD controller. Finally a control law is derived for a sliding mode controller. The performances of the different controllers are evaluated for step inputs of 1000, 200 and 1500 pm respectively. The simulated PD and fuzzy logic responses showed remarkable correlation as well as the experimental results. The sliding mode controller is simulated and intuitively optimised. The simulated and practical responses of the sliding mode controller also showed good correlation. Some differences in the response are attributed to implementation discrepancies. The controllers are compared in terms of the equivalent stiffness and damping of the different systems. The ITAE performance index is used as an additional comparative criterion and identified the sliding mode controller as superior to the linear controllers. This project emphasises the importance of accurate modelling. Future work will involve delinearization of the fuzzy logic controller and a specialised study on sliding mode control.