Identification of Force-Displacement and Force-Current Factors in an Active Magnetic Bearing System

Abstract

Conventional magnetic bearings are usually operated in differential driving mode. This configuration makes it possible to generate both positive and negative forces. The linearization of this force results in force-current and force-displacement factors, which make the control design easier as it can exploit all the tools of linear control theory. However, the control performance can deteriorate when these factors do not closely match the real plant. This paper presents a procedure to identify the force-displacement and force-current factors in an active magnetic bearing (AMB) system using grey-box modeling. These force factors are obtained for different operating points defined by a range of bias currents. Since the AMB has an unstable open loop nature, a position Proportional-Integral-Derivative (PID) controller is implemented to stabilize the plant. Experimental data are collected exciting the system with a persistent disturbance current. The models tuned with the identified parameters were able to describe the plant dynamics with improved accuracy. The set of identified parameters can be exploited in model-based control approaches by varying energy-efficiently the bias current.