The Foundation of Electromagnets Based Active Vibration Control

Abstract

Turbines, pumps, compressors, blowers and all rotating machinery in general, is commonly used in process industry, including machining tools, power generation, as well as aircraft and marine propulsion among the most important industrial applications. Mass imbalance is commonly responsible for rotating machinery vibration. When the principal axis of inertia of the rotor is not coincident with its geometric axis imbalance occurs. Nevertheless there are some more causes for rotating machinery vibration such as operation near resonant frequencies, critical speeds and so on. Higher speeds cause much greater centrifugal imbalance forces, and the current trend of rotating equipment toward higher power density clearly leads to higher operational speeds. For instance, speeds approaching 35,000 rpm are common in machining applications. Therefore, vibration control is essential in improving machining surface finish; achieving longer bearing, spindle, and tool life in high-speed machining; and reducing the number of unscheduled shutdowns. A great cost savings for high-speed turbines, compressors, and other turbomachinery used in petrochemical and power generation industries can be realized using vibration control technology. Passive and active vibration control (AVC) techniques of rotating machinery are being used. It is well established that the vibration of rotating machinery can be reduced by introducing passive or active devices into the system. Although an active control system is usually more complicated than a passive vibration control scheme, an AVC technique has many advantages over a passive vibration control technique. In (Fuller et al., 1996) it is shown that AVC is more effective than passive vibration control in general. Furthermore, the passive vibration control is of limited use if several vibration modes are excited. Finally, because the active actuation device can be adjusted according to the vibration characteristic during the operation, the active vibration technique is much more flexible than passive vibration control. There are two major categories in AVC techniques for rotating machinery: • Direct active vibration control (DAVC) techniques in which directly apply a lateral control force to the rotor. • Active balancing techniques which adjust the mass distribution of a mass redistribution actuator. Active balancing isn’t under the scope of this chapter. The control variable in DAVC techniques is a lateral force generated by a force actuator based on a magnetic bearing. The advantage of DAVC techniques is that the input control force to the system can be changed according to vibration characteristics.