Session H20 - Gravitational Radiation - Experiment.
MIXED session, Sunday morning, April 30
102A, LBCC
The Seismic Attenuation System (SAS) has to be actively controlled over a frequency band of up to several Hz in order to damp its own rigid body modes (inertial damping), to generate DC local and global positioning, and to reduce residual rms motion to acquire the locking of the interferometer. The control system incorporates signals from local sensors (for displacement and acceleration) and the interferometer and generates adequate feedback signals for various actuators on different levels of the SAS chain. The control system is organized in a hierarchical scheme. With a large dynamic range at higher stages of the SAS, it damps internal modes of the system which minimizes requirements for the suspension control. The control system is a Multiple Input and Multiple Output (MIMO) that can be separated to simple Single Input and Single Output (SISO) feedback loops by using fast DSP boards. SAS controls are limited to a frequency band well below 10 Hz, to avoid noise injection in the gravitational wave band. Above this frequency, the SAS behaves as a completely passive seismic attenuator. According to simulated SAS performance based on measured seismic noise, achievable residual r.m.s. motion of SAS is a few tens of nm above 100 mHz. A similar system for VIRGO has already achieved 50 nm r.m.s. displacement.