Active seismic isolation using interferometric inertial sensors

The transmission of ground motion to sensitive equipment can affect their performance or their duty cycle. To overcome this limitation, passive and active isolation solutions have been developed. When it is supported by springs, a system is isolated passively in a frequency range above its resonance frequency. On the other hand, active isolation consists in sensing the motion of the payload to isolate and canceling this motion by injecting a force that opposes this motion. To increase the performance, passive and active solutions can be combined.The goal of this research is to isolate a six degree-of-freedom (d.o.f.) platform from ground motion in the low-frequency range, i.e. between 10 mHz and 10 Hz. At low frequency, the performance of the active isolation stage is limited by the sensor resolution. To overcomethis limitation, one can use inertial sensors whose inertial mass’ motion is measured by an interferometric readout.In this thesis, the characterization of a horizontal inertial sensor (HINS) and a vertical inertial sensor (VINS) developed is presented. The Michelson interferometer used to sense the inertial mass is studied with a numerical model; the current resolution is limited by the optics too sensitive to their mounting’s motion and by the signal drift at low frequency. The mechanical system is made of a pendulum. By deriving the equations of motion of the system, the sensor’s dynamics can be studied; Inertial sensors sense a coupled signal between rotation and translation. Because of this coupling, the dynamics include a pair of complex or real zeroes at low frequency, below the resonance frequency of the inertial system.To subtract the spurious tilt motion from the sensor’s signal, a liquid sensor is under development. The inclination is measured by comparison to the horizontal surface of liquid Mercury sealed inside a container. It has been characterized experimentally to study its sensitivity, 306.75 V/rad, and its resolution on the order of 5 10−5 rad/pHz. The current version is too noisy principally because the viscosity of the reflective liquid is not large enough and hence, the reference surface moves too much with ground motion.The effect of the tilt-translation coupling on inertial control is studied on a 2 d.o.f. model; when applying inertial control, the motion of the payload becomes coupled with the ground inclination at low frequency due to the sensor’s dynamics. Solutions to cope with coupling have been investigated: projecting the system in a decoupled frame (centralized projection and singular value decomposition (SVD)), closing a first loop to control the rotation of the payload or sensing the inclination and subtracting it from the inertial sensor signal. The last two methods gave the most promising results.Finally, a 6 d.o.f. platform has been assembled. It consists of a hexagonal table with three pairs of vertical and horizontal inertial sensors. The table lays on three isolators containing voice coil actuators. The effect of inertial control on a multiple input/multiple output (MIMO) system is studied experimentally and the solutions tested numerically on the 2 d.o.f. model are extended to the full isolation stage. The platform could reduce of one order of magnitude the amplitude of the payload’s motion between 0.1 Hz and 10 Hz in the vertical direction and between 0.1 Hz and 1 Hz in the horizontal direction.Doctorat en Sciences de l'ingénieur et technologie