Magnetic levitation technique for active vibration control

This chapter presents an application of zero-power controlled magnetic levitation for active vibration control. Vibration isolation are strongly required in the field of high-resolution measurement and micromanufacturing, for instance, in the submicron semiconductor chip manufacturing, scanning probe microscopy, holographic interferometry, cofocal optical imaging, etc. to obtain precise and repeatable results. The growing demand for tighter production tolerance and higher resolution leads to the stringent requirements in these research and industry environments. The microvibrations resulted from the tabletop and/or the ground vibration should be carefully eliminated from such sophisticated systems. The vibration control research has been advanced with passive and active techniques. Conventional passive technique uses spring and damper as isolator. They are widely used to support the investigated part to protect it from the severe ground vibration or from direct disturbance on the table by using soft and stiff suspensions, respectively (Haris & Piersol, 2002; Rivin, 2003). Soft suspensions can be used because they provide low resonance frequency of the isolation system and thus reduce the frequency band of vibration amplification. However, it leads to potential problem with static stability due to direct disturbance on the table, which can be solved by using stiff suspension. On the other hand, passive systems offer good high frequency vibration isolation with low isolator damping at the cost of vibration amplification at the fundamental resonance frequency. It can be solved by using high value of isolator damping. Therefore, the performance of passive isolators are limited, because various trade-offs are necessary when excitations with a wide frequency range are involved. Active control technique can be introduced to resolve these drawbacks. Active control system has enhanced performances because it can adapt to changing environment (Fuller et al., 1997; Preumont, 2002; Karnopp, 1995). Although conventional active control system achieves high performance, it requires large amount of energy source to drive the actuators to produce active damping force (Benassi et al., 2004a & 2004b; Yoshioka et al., 2001; Preumont et al., 2002; Daley et al., 2006; Zhu et al., 2006; Sato & Trumper, 2002). Apart from this, most of the researches use high-performance sensors, such as servo-type accelerometer for detecting vibration signal, which are rather expensive. These are the difficulties to expand the application fields of active control technique. 3