A control model of active magnetic bearings

Active magnetic bearings (AMBs) support a rotating shaft, inside a stator, contactless. Their major benefit in respect to hydrodynamic journal bearings is the possibility of operating in much higher rotational speeds (above 20.000 RPM). This is due to the fact that, their angular velocity is only limited by the strength of the shaft material and always they have stabilized operation without any mechanic contact. Also, AMBs are resulting in long life time, without the use of any lubrication system, eliminating thus the complexity of the lubricant network and promoting green operation of the rotating machines. Furthermore, the electromagnetic bearings are active elements that allow the measurement and the control of the position of the shaft, targeting in most accurate equilibrium positions, in terms of the shaft operation. Therefore, a suitable control system is required for a magnetic bearing, to exploit the above advantageous operational characteristics. In this study, the model of the control system of a Magnetic Bearing in terms of sensors and power amplifiers is presented and its design guidelines are explicitly described. As a first step, the non linear equation of the movement of the shaft inside the magnetic bearing, in one direction (y-axis), is introduced. Then, the linearization of this particular equation in a specific equilibrium point is performed and presented. Consequentially, the linear equation is transformed via Laplace method and the model of the Active Magnetic Bearing is finally developed. In addition to these, the sensor and the power amplifiers are modeled, to simulate the AMB system as a whole. The Matlab software is used to simulate the components that the AMB is consisted of. The P, I and D gains are the result of this simulations, which are produced via robust control method, in Single-Input-Single-Output (SISO) tool module of Matlab. At the end, the Simulink module of Matlab is energized to check the control characteristics of the developed AMB system, taking into account the non-linear equations of the electromagnetic forces and the saturation of the coils. The rotor response, the overshoot, the rotor centre vibration are controlled using the above produced P, I and D gains. Several plots are constructed and presented to show the AMB control possibility with the aforementioned methodology.