Active hard mount vibration isolation for precision equipment

Floor vibrations and acoustic excitation may limit the performance of precision equipment, that is used for example to produce computer chips or to make images of very tiny structures. Therefore, it is common to mount a vibration isolator in the suspension of such equipment to isolate it from these vibration sources. Most often, a vibration isolator with a low suspension stiffness is used to limit the transmission of floor vibrations onto the suspended equipment. However, a low suspension stiffness introduces problems with leveling of the equipment and it increases its susceptibility to acoustic excitation. The objective of this thesis is to develop an alternative solution based on an active hard mount vibration isolator, which provides a much stiffer suspension such that the problems mentioned before can be circumvented. An active control system, that consists of sensors, actuators and a controller, is used to keep a low transmission of floor vibrations. An additional advantage is that the damping of internal modes of the equipment can be increased, such that its accuracy is improved. Several feedback control strategies for the active hard mount vibration isolator are presented. In both models and experiments it is shown that the best control strategy is using a combination of acceleration and force sensors. Formulas for adjusting the control parameters to obtain the desired vibration isolation performance have been derived. Furthermore, a demonstration setup of a six-axes active hard mount vibration isolator is developed. Unfortunately, due to some hardware problems, the usable control bandwidth is limited and the noise level of the sensors is too high, which results in a vibration isolation performance that is less than desired. Despite these problems, the transmission of floor vibrations is reduced by a factor 18, while simultaneously the suspension stiffness is about 100 times higher as compared to a vibration isolator with a low suspension stiffness. In addition, the damping of the internal modes is increased significantly. Further research should focus on improving the mechanical design of the active hard mount vibration isolator, such that the control bandwidth can be increased, and on using sensors with ultra-low noise levels.