Six-degree of freedom active pneumatic table based on time delay control technique

Abstract

Precision equipment is sensitive in general to both horizontal and vertical ground vibrations and, thus, equipment manufacturers specify limits on the vibrations of the floor so that such equipment can be operated satisfactorily. When the floor vibrations are unacceptably high, various types of passive or active vibration isolation tables can be used. A three-degree of freedom active pneumatic table with time delay control was developed for control of heaving, pitching, and rolling motions in a previous study by the authors. The isolation performances achieved were very good for both ground and table excitations in the vertical directions. Limitations of the three-degree of freedom active pneumatic table, however, were that it is vulnerable to excitations of the floor and table in the horizontal directions. Hence, a six-degree of freedom active pneumatic table was required in order to tackle ground and table excitations in the horizontal as well as vertical direction. In this paper, a study on the six-degree of freedom active pneumatic table supported by four air-springs for both vertical and horizontal vibration control is presented. Its performance is evaluated by ground and table excitation tests. The physical meaning of the singular value decomposition approach in transmissibility analysis on a multi-input/multi-output vibration isolation system is presented using a simple two-degree of freedom example and applied to the actual six-degree of freedom active pneumatic table. It is also shown that settling time in the case of table excitations in both vertical and horizontal directions can be effectively reduced by the proposed six-degree of freedom control.