Speaker
Description
Significant efforts have been made to make a conceptional design for the test mass suspensions for the low frequency interferometer of the Einstein Telescope. A complete model of the suspension, including an actively controlled 6DoF seismic isolation platform, inverted pendulum system and a cascade of pendulum stages is modeled in Femto. The presentation will focus on the following aspects.
Modeling and Control
A full implementation of the control of the platform, inverted pendulum and local damping of the suspension chain shows that a plant with nicely damped eigenmodes can be achieved, without injection of control noises into the sensitivity band of ET, above 3Hz. Several important aspects are modelled in detail, amongst which:
- Back-reactions from actuation at suspension stages to actuation cages or preceding stages,
- couplings between all elements of the suspension are taken into account and eigenmodes of the chain are verified to not disturb control of the platform in a significant manner,
- amplification of sensor noise due to damping action of the chain at high frequencies is limited,
- sensor placement to effectively subtract parasitic 'cage noise' due to lever arm effects that are inevitable when using relative sensing between suspension stages.
Currently, the controlled suspension design is used to optimize the global control distribution over the payload stages. This control system is responsible to 'lock' the Fabry Pérot cavities to their operating point.
Towards Computer Aided Design
Current efforts are also focused on utilizing controller optimization techniques to automatically design feedback controllers and filters. By doing so it becomes easier and moreover much quicker to assess whether a suspension design design is able to achieve the requirements for ET. This can be combined later with modeling and parameter optimization in Femto, this gives us a solid base to iterate efficiently over suspension designs to get to a more optimal concept with every step.
Outlook
Having modeled a test-mass suspension that is also controlled, can help to define requirements and strategies for the control of the cavity modes and optical spring effects. To effectively tackle these effects, the modelling and simulations that have been done so far are ready to be combined with optical simulations to get closed to a more integrated design of test-masses that work together to form the main interferometer.