Active Control of Rotating Machinery Noise Through Use of Magnetic Bearings

This paper investigates magnetic bearings as an actuator for noise control. The apparatus used consists of a DC motor connected to a fan blade by a short, rigid shaft supported radially and axially by magnetic bearings. The bearings provide position control of the shaft, and thus the fan blade as well. This position control was used to vibrate the fan blade as an effective speaker for producing the secondary sound in an active sound control scheme. Due to the proximity of this secondary noise source to the primary noise sources the motor and fan blade good control of low-frequency noise can be achieved at all points in space. For this project, the objective was tonal noise control at frequencies corresponding to blade rate, as indicated by tachometer feedback. Noise control algorithms such as the least mean squares algorithm were implemented on a dedicated digital signal processor, with the output translated into position commands for the magnetic bearing controller. The use of magnetic bearings as an actuator for noise control is demonstrated, with tonal noise reduction of up to 6 dB achieved. This project has application to rotating machinery and marine propulsion systems.