Non Linear Control of Active Magnetic Bearings: Digital Implementation

The aim of this paper is to present a non linear control scheme synthesis and its digital implementation. The process under consideration is a rotating shaft magnetically suspended by means of eight electromagnets located by pairs in order to control four degrees of freedom of the shaft. A passive thrust stabilises the motion along the main axis of the shaft. The problem is to drive actuator inputs which can be the currents or voltages of each electromagnet. In this paper we focus on the non linear and multivariable features of the model : it is why we don't introduce premagnetisation currents and we don't reduce the model of the system to its linearized form around an equilibrium point. The description of the control scheme synthesis is decomposed into two parts. The first one is devoted to the analysis of the continuous control scheme, while the second presents the transformation of this scheme in a digital version which can be implemented in a computer. Then some experimental results are presented. These correspond to the behaviour of a pilot which is used to experiment the control laws. The analysis of these data raises some new questions about disturbances or modeling improvements, which are discussed in the conclusion.