A Comparison of Analog and Digital Controls for Rotor Dynamic Vibration Reduction Through Active Magnetic Bearings

Abstract

Active magnetic bearings are implemented using analog and digital controllers to achieve vibration reduction for multimass flexible rotors. Various models are developed for the rotor-bearing system, and the first three critical speeds (resonant frequencies) are shown to be unaffected by inclusion of the higher order shaft dynamics in the model. Higher order rotor-bearing models reveal the presence of “shaft modes,” the excitation of which is a function of the position of the magnetic bearing proximity probe (these modes are effectively damped by the flexible coupling employed in the test apparatus). Rotor dynamic behavior is investigated for various analog and digital controllers. Rotor response in the presence of proportional-derivative control is similar for both analog and digital control. Higher order digital control algorithms (second derivative and integral) affect the rotor response in a frequency dependent manner: Second derivative feedback is effective at reducing third mode vibration, and integral feedback, while rejecting any steady-state rotor position error, slightly accentuates the vibration at the first critical speed. Increasing the sampling rate of the digital controller has a similar effect to increasing the amount of second derivative feedback employed.