A proposal of control system for natural frequency reducing method of anti-vibration apparatus

Abstract

In the field of precision positioning such as semiconductor exposure apparatus, weak disturbance vibration from the floor influences processing accuracy. In recent years, as the precision of the device increases, the vibration allowance value becomes more severe, and performance improvement of the vibration isolator is required. In order to improve the performance, there are researchs reducing natural frequency by the frequency characteristics of the vibration transmissibility from the floor to the vibration isolator. Reducing natural frequency and decreasing stiffness are equivalent, and main target was stiffness generated by compressed air of air spring. In addition, rubber bellows of air spring is also targeted for further performance improvement. We already proposed a method combining the two stiffness separately decreasing method. However, decreasing stiffness simultaneously destabilized the device, then it could not be completely decreased. In this paper, as a new stabilization method, stiffness is completely decreased only in the operation frequency band. Since the low frequency has positive rigidity, the localization is maintained. We demonstrated the reducing natural frequency by the experimental result. When stiffness was completely decreased, the servo stiffness which had not been focused in the past became dominant in determining the natural frequency. Therefore, an appropriate design of the servo stiffness is required for further reducing natural frequency. To satisfy this requirement, we calculated the relationship between natural frequency and servo stiffness and gave an index for appropriate design.

Keywords

• vibration isolation
• vibration control
• servo mechanism
• natural frequency