Last stage control and mechanical transfer function measurement of the VIRGO suspensions

The automatic control of the suspended mirrors is a major task in operating an interferometric gravitational waveantenna. To reach the extreme sensitivity required for this kind of detector, an accurate alignment and a stable locking of the interferometer on its working point are crucial. The solution of this problem is particularly complex in the case of a multistage pendulum, such as the suspension system for seismic isolation adopted in VIRGO. A precise knowledge of the suspension mechanical transfer functions (TFs) for different forces applied in the control servo-loops represents essential information to reach the goal. In this article, we describe the apparatus we developed to measure the VIRGO suspension TF and we report the results thus obtained on full-scale suspensions at the VIRGO site. Preliminary results for the implemented control system of the last suspension stage are also presented.