Nano precision instruments with nanometer resolution are susceptible to disturbance transmitted mostly from ground vibration and mechanical vibration due to movements of the measuring instrument itself. These kinds of vibrations which are submicrometer level and low frequency distract both precision and accuracy of measuring instruments.
So, precision instruments require six degree of freedom active vibration isolation system. In six degree of freedom active vibration isolation system passive isolator and control algorithm are very important factors of performance improvement. The system of this sort has coupling and system uncertainties due to change payload mass.
Six degree of freedom active vibration isolation system used in this system has four passive mount and six VCM actuators to transmit force to six axes and six accelerometers to measure six axes acceleration of payload.
In this paper, passive isolation system is designed to support payload and attenuate high frequency. For system control, PI controller which is easy to design and $H_\infinity$ controller which is robust to uncertainties and disturbances are designed and proposed to control active isolator and applied to six degree of freedom active vibration isolation system. And then, performance of PI controller is compared with $H_\infinity$ controller.
In conclusion, $H_\infinity$ controller is more compatible with six degree of freedom active vibration isolation system which has coupled axis and system uncertainty.
