Development and test of sensors and actuators 
to control a Michelson interferometer 
suspended by means of a multipendular system
Creators: name: Persichetti, Gianluca
Publisher: name: Università degli Studi di Napoli Federico II
Year: 2010

description: The purpose of the VIRGO experiment is to detect gravitational waves produced by astrophysical sources in a frequency range between 10 Hz and some KHz. 
VIRGO is the unique interferometric detector devoted to detect signals below 50 Hz and one of its main goals is to extend the low frequency detection threshold down to a few Hz (to enlarge the potentially detectable sources number). In this frequency range, the seismic noise limits the detector sensitivity. One of the most efficient Seismic Attenuation System (SAS) for gravitational waves interferometers is a chain of pendula suspended from a very low-frequency stage called Inverted Pendulum (IP). Each interferometer optical component is supported by means of this particular suspension system. In the Naples VIRGO laboratory a simplified prototype of this interferometric detector is building. Thanks to this prototype interferometer it will be possible to develop and test new sensors as monolithic accelerometers and optical position sensors, new actuation systems to employ in the mirror control (electrostatic actuators) and designing new control system for the interferometer. 
Besides the vibration isolation offered by the IP, residual motion of the mirror and of the pendula chain stages have to be damped. For this purpose a feedback system, using inertial sensors (accelerometers) and position sensors (LVDT) has been designed.   
An original experimental procedure for determining the IP feedback control has been defined and tested on the system. 
To improve feedback control system performance, new monolithic accelerometers has been developed in collaboration with the University of Salerno. Such a kind of new inertial sensors seems to be very promising and their use is not limited on inertial control but they can find employ as very low frequency seismometers. The accelerometers, shaped with precision machining and electric-discharge machining, are a very compact instrument, very sensitive in the low-frequency seismic noise band, with a very good immunity to environmental noises. 
A characterization of this new inertial sensors has been performed. One of the most important parameter to determine the performance of this sensor is the mechanical quality factor. To this purpose, a vacuum chamber has been set up and mechanical quality factor measurements has been performed on different accelerometers model. In particular, one of the model tested  has shown exceptional quality factor. Another parameter which determines the overall performance is the frequency resonance. The accelerometers are expressly designed to change this frequency because of the presence of a tuning mass. The measurements have shown very low frequency resonance values. 
Additional research activity was related to the characterization of an alternate biased electrostatic actuator to be employed for the control of the suspended mirror. Electrostatic actuators (EA) are the most promising devices for mirror control for next generation of interferometric gravitational wave detectors. 
Characterization of electrostatic actuation force in air and in vacuum has been performed. The measures has been compared with a theoretical model. An efficient technique to investigate the spurious charge presence on the mass has been determined. This original procedure shows that, by driving the EA with a modulated bias, it is possible to minimize the unwanted additional force generated by the spurious charges.