A six-degrees-of-freedom platform for deci-hertz active isolation using optical inertial sensors

This paper addresses a theoretical control approach and its corresponding experimental validation for low-frequency active damping and isolation of a six-degree-of-freedom platform using high-resolution inertial sensors. Six vacuum-operating inertial sensors are placed on top of the platform to actively control it. Three of them measure displacements in horizontal directions and three in vertical directions. The resonance frequencies of the vertical and horizontal sensors range between 0.3 and 0.7 Hz with a resolution of <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mn>2</mml:mn> <mml:mo>×</mml:mo> <mml:mn>1</mml:mn> <mml:msup> <mml:mrow> <mml:mn>0</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>13</mml:mn> </mml:mrow> </mml:msup> <mml:mspace width="0.3333em"/> <mml:mtext>m</mml:mtext> <mml:mo>/</mml:mo> <mml:msqrt> <mml:mrow> <mml:mtext>Hz</mml:mtext> </mml:mrow> </mml:msqrt> </mml:math> at 1 Hz for both types of sensors. Sensor signals are fed back into six voice coil actuators (three horizontal and three vertical) mounted below the platform. Actuators and sensors are placed in a quasi-collocated architecture facilitating the controllability of the plant. The platform (with resonance frequencies ranging between 1 and 10 Hz) is actively isolated by up to two orders of magnitude between 0.1 and 10 Hz, yielding a final overall displacement RMS value below 100 nm at its center of mass from 0.3 Hz onward.