Identification and Control of a Magnetically Levitated Table

A new vibration isolation system is developed. A table is levitated by an active magnetic bearing and isolated from any external vibration. Subspace identification methods are used to find a state-space model for this system from input-output data. A PID controller is designed to levitate the table, and a random signal is used to excite the system in addition to the controller inputs. This paper points out some problems when subspace methods are used for identification of closed-loop systems. Although three subspace methods are applied to the present problem, only one of them works well. Two subspace methods are not able to find a model with order of a reasonable value. We present one of the reasons why those subs'pace methods do not work.